|Did you leave a blank space between assoc and .exe ?|
If you typed in the command assoc.exe, this is incomplete because it does not incorporate an extension to reset. The correct one is assoc.exe .exe=exefile ( or assoc .exe=exefile. The command may be typed in without its own extension.). I think you see the difference. Pls see this MS KB Article for further info.
If you want to see what this command does in Win 7, open a command prompt and type in
ASSOC /? (there is a blank space before the slash)
and press enter.