Start -> Settings -> Control Panel -> System -> Advanced -> Performance (Settings) -> Advanced -> Virtual Memory (Change). As AgentSmith said, it's a good idea to put your virtual memory on a different drive from your OS files. And if you increase your VM too much, Windows might try to use it regardless whether they need it, and it will slow things down. Microsoft recommends 1.5 times RAM. I've got one gig, so my VM is 1.5 GB.

-- erlik