Get a USB stick that supports readyboost, these are faster than the usual usb sticks on the market, then stick it into the usb port on your pc.

The autoplay window will show you a few options, click on Speed up my system (using Windows ReadyBoost) - if your usb stick is the right speed, you should be able to select the amount of memory to allocate to Vista in order to speed it up. It is reccomended that you use ReadyBoost on systems with 1gb of ram and above, and that you allocate around the same amount of memory for ReadyBoost that you have of RAM.

Go to this page for a more thorough guide on USB Memory .