If you don’t need it, disable Windows Index search and Restore points. Hard drive is probably the worst slowdown. Try to tweak your windows as if it were a real box you have with limited hardware. Removing unnecessary services also helps a lot and keeps windows more quiet.
There are various guides around the net (google is your friend) on what services you can “safely” disable to get a performance boost.
Also search on Stack Overflow for a few Visual Studio tweaks (there are a few interesting threads about it there); it all helps a lot. If you have an external FW800 or faster drive, moving the VM there also helps balancing the load between the two slowest pieces of hardware. Unless, of course, you have a SSD :)