Windows manages the size of it's virtual memory dynamically so normally you wouldn't need to mess with it.
But if you insist on having a play then right click on My Computer and pick properties. Pick the Advanced tab and you then want click the Settings button under Performance.
Now pick the Advanced tab and then click the change button under Virtual Memoery at the bottom.
A good rule of thumb is to set it at one and a half times the size of your RAM.