<s>
A	O
registry	B-Application
cleaner	I-Application
is	O
a	O
class	O
of	O
third-party	B-General_Concept
utility	B-Application
software	I-Application
designed	O
for	O
the	O
Microsoft	B-Application
Windows	I-Application
operating	I-Application
system	I-Application
,	O
whose	O
purpose	O
is	O
to	O
remove	O
redundant	O
items	O
from	O
the	O
Windows	B-Protocol
Registry	I-Protocol
.	O
</s>
<s>
Registry	B-Application
cleaners	I-Application
are	O
not	O
supported	O
by	O
Microsoft	O
,	O
but	O
vendors	O
of	O
registry	B-Application
cleaners	I-Application
claim	O
that	O
they	O
are	O
useful	O
to	O
repair	O
inconsistencies	O
arising	O
from	O
manual	O
changes	O
to	O
applications	O
,	O
especially	O
COM-based	O
programs	O
.	O
</s>
<s>
The	O
effectiveness	O
of	O
Registry	B-Application
cleaners	I-Application
is	O
a	O
controversial	O
topic	O
.	O
</s>
<s>
Due	O
to	O
the	O
sheer	O
size	O
and	O
complexity	O
of	O
the	O
Registry	O
database	O
,	O
manually	O
cleaning	O
up	O
redundant	O
and	O
invalid	O
entries	O
may	O
be	O
impractical	O
,	O
so	O
Registry	B-Application
cleaners	I-Application
try	O
to	O
automate	O
the	O
process	O
of	O
looking	O
for	O
invalid	O
entries	O
,	O
missing	O
file	O
references	O
or	O
broken	O
links	O
within	O
the	O
Registry	O
and	O
resolving	O
or	O
removing	O
them	O
.	O
</s>
<s>
The	O
correction	O
of	O
an	O
invalid	O
Registry	B-Protocol
key	I-Protocol
(	O
such	O
as	O
one	O
or	O
more	O
that	O
remain	O
after	O
uninstallation	O
of	O
a	O
program	O
application	O
)	O
can	O
provide	O
some	O
benefits	O
;	O
but	O
the	O
most	O
voluminous	O
will	O
usually	O
be	O
quite	O
harmless	O
,	O
obsolete	O
records	O
linked	O
with	O
COM-based	O
applications	O
whose	O
associated	O
files	O
are	O
no	O
longer	O
present	O
.	O
</s>
<s>
Some	O
Registry	B-Application
cleaners	I-Application
make	O
no	O
distinction	O
as	O
to	O
the	O
severity	O
of	O
the	O
errors	O
,	O
and	O
many	O
that	O
do	O
may	O
erroneously	O
categorize	O
errors	O
as	O
"	O
critical	O
"	O
with	O
little	O
basis	O
to	O
support	O
it	O
.	O
</s>
<s>
It	O
is	O
not	O
always	O
possible	O
for	O
a	O
third-party	B-General_Concept
program	O
to	O
know	O
whether	O
any	O
particular	O
key	O
is	O
invalid	O
or	O
redundant	O
.	O
</s>
<s>
A	O
poorly	O
designed	O
Registry	B-Application
cleaner	I-Application
may	O
not	O
be	O
equipped	O
to	O
know	O
for	O
sure	O
whether	O
a	O
key	O
is	O
still	O
being	O
used	O
by	O
Windows	O
or	O
what	O
detrimental	O
effects	O
removing	O
it	O
may	O
have	O
.	O
</s>
<s>
This	O
may	O
lead	O
to	O
loss	O
of	O
functionality	O
and/or	O
system	O
instability	O
,	O
as	O
well	O
as	O
application	B-General_Concept
compatibility	I-General_Concept
updates	I-General_Concept
from	O
Microsoft	O
to	O
block	O
problematic	O
Registry	B-Application
cleaners	I-Application
.	O
</s>
<s>
The	O
Windows	B-Device
Installer	I-Device
CleanUp	I-Device
Utility	I-Device
was	O
a	O
Microsoft-supported	O
utility	O
for	O
addressing	O
Windows	B-Application
Installer	I-Application
related	O
issues	O
.	O
</s>
<s>
Registry	B-Application
cleaners	I-Application
have	O
been	O
used	O
as	O
a	O
vehicle	O
by	O
a	O
number	O
of	O
trojan	O
applications	O
to	O
install	O
malware	O
,	O
typically	O
through	O
social	O
engineering	O
attacks	O
that	O
use	O
website	O
pop-up	O
ads	O
or	O
free	O
downloads	O
that	O
falsely	O
report	O
problems	O
that	O
can	O
be	O
"	O
rectified	O
"	O
by	O
purchasing	O
or	O
downloading	O
a	O
Registry	B-Application
cleaner	I-Application
.	O
</s>
<s>
The	O
rogue	O
security	O
software	O
"	O
WinFixer	O
"	O
including	O
Registry	B-Application
cleaners	I-Application
has	O
been	O
ranked	O
as	O
one	O
of	O
the	O
most	O
prevalent	O
pieces	O
of	O
malware	O
currently	O
in	O
circulation	O
.	O
</s>
<s>
Rogue	O
Registry	B-Application
cleaners	I-Application
are	O
often	O
marketed	O
with	O
alarmist	O
advertisements	O
that	O
falsely	O
claim	O
to	O
have	O
pre-analyzed	O
your	O
PC	O
,	O
displaying	O
bogus	O
warnings	O
to	O
take	O
"	O
corrective	O
"	O
action	O
;	O
hence	O
the	O
descriptive	O
label	O
"	O
scareware	O
"	O
.	O
</s>
<s>
In	O
October	O
2008	O
,	O
Microsoft	O
and	O
the	O
Washington	O
attorney	O
general	O
filed	O
a	O
lawsuit	O
against	O
two	O
Texas	O
firms	O
,	O
Branch	O
Software	O
and	O
Alpha	O
Red	O
,	O
producers	O
of	O
the	O
"	O
Registry	B-Application
Cleaner	I-Application
XP	B-Application
"	O
scareware	O
.	O
</s>
<s>
-	O
REGISTRY	O
DAMAGED	O
AND	O
CORRUPTED	O
"	O
,	O
before	O
instructing	O
users	O
to	O
visit	O
a	O
web	O
site	O
to	O
download	O
Registry	B-Application
Cleaner	I-Application
XP	B-Application
at	O
a	O
cost	O
of	O
$39.95	O
.	O
</s>
<s>
However	O
this	O
is	O
less	O
of	O
an	O
issue	O
with	O
NT-based	B-Device
operating	B-General_Concept
systems	I-General_Concept
(	O
including	O
Windows	B-Application
XP	I-Application
and	O
Vista	B-Application
)	O
,	O
due	O
to	O
a	O
different	O
on-disk	O
structure	O
of	O
the	O
Registry	O
,	O
improved	O
memory	O
management	O
,	O
and	O
indexing	B-Data_Structure
.	O
</s>
<s>
Conversely	O
,	O
defragmenting	B-Application
the	O
underlying	O
Registry	O
files	O
(	O
e.g.	O
</s>
<s>
using	O
the	O
free	O
Microsoft-supported	O
PageDefrag	B-Application
tool	O
)	O
,	O
rather	O
than	O
attempting	O
to	O
clean	O
the	O
Registry	O
'	O
s	O
contents	O
,	O
has	O
a	O
measurable	O
benefit	O
and	O
has	O
therefore	O
been	O
recommended	O
in	O
the	O
past	O
by	O
experts	O
such	O
as	O
Mark	O
Russinovich	O
.	O
</s>
<s>
Most	O
Registry	B-Application
cleaners	I-Application
cannot	O
repair	O
scenarios	O
such	O
as	O
undeletable	O
Registry	B-Protocol
keys	I-Protocol
caused	O
by	O
embedded	O
null	O
characters	O
in	O
their	O
names	O
;	O
only	O
specialized	O
tools	O
such	O
as	O
the	O
RegDelNull	O
utility	O
(	O
part	O
of	O
the	O
free	O
Sysinternals	O
software	O
)	O
are	O
able	O
to	O
do	O
this	O
.	O
</s>
<s>
A	O
Registry	B-Application
cleaner	I-Application
cannot	O
repair	O
a	O
Registry	O
hive	O
that	O
cannot	O
be	O
mounted	O
by	O
the	O
system	O
,	O
making	O
the	O
repair	O
via	O
"	O
slave	O
mounting	O
"	O
of	O
a	O
system	O
disk	O
impossible	O
.	O
</s>
<s>
Automated	B-General_Concept
System	I-General_Concept
Recovery	I-General_Concept
,	O
from	O
a	O
"	O
last	O
known-good	O
"	O
boot	O
menu	O
,	O
by	O
re-running	O
setup	O
or	O
by	O
using	O
System	O
Restore	O
)	O
.	O
</s>
<s>
"	O
Last	O
known-good	O
"	O
restores	O
the	O
last	O
system	B-Protocol
Registry	I-Protocol
hive	O
(	O
containing	O
driver	O
and	O
service	O
configuration	O
)	O
that	O
successfully	O
booted	O
the	O
system	O
.	O
</s>
<s>
Registry	B-Application
cleaners	I-Application
are	O
likewise	O
not	O
designed	O
for	O
malware	O
removal	O
,	O
although	O
minor	O
side-effects	O
can	O
be	O
repaired	O
,	O
such	O
as	O
a	O
turned-off	O
System	O
Restore	O
.	O
</s>
<s>
A	O
Registry	B-Application
cleaner	I-Application
is	O
of	O
no	O
use	O
for	O
cleaning	O
Registry	O
entries	O
associated	O
with	O
a	O
virtualised	B-Operating_System
application	I-Operating_System
since	O
all	O
Registry	O
entries	O
in	O
this	O
scenario	O
are	O
written	O
to	O
an	O
application-specific	O
virtual	O
Registry	O
instead	O
of	O
the	O
real	O
one	O
.	O
</s>
<s>
In	O
general	O
,	O
even	O
if	O
Registry	B-Application
cleaners	I-Application
could	O
be	O
arguably	O
considered	O
safe	O
in	O
a	O
normal	O
end-user	O
environment	O
,	O
they	O
should	O
be	O
avoided	O
in	O
an	O
application	B-Operating_System
virtualization	I-Operating_System
environment	O
.	O
</s>
