Installing Hyper-V and Virtual Server 2005 R2 SP1 (or any version) on the same physcial server is not a recommended practice. You might ask why?
Hyper-V uses hypervisor technology that requires and abstracts the hardware virtualization support in the processor from the remaining applications and partitons. While you can install Virtual Server 2005 R2 SP1 in the parent partition of a Hyper-V host, Virtual Server will not see the virtualization processor extensions while the hypervisor is running. This means that Virtual Server 2005 R2 SP1 would run in emulation mode and drastically impact performance.
If you need to run both products, it is recommended to use separate servers. It is possible to disable the hypervisor and get Virtual Server 2005 to see the virtualization extensions, but it requires a reboot.