If you have a spare U.S.B. flash drive with at least 256 megabytes of free space on it, you can use it to give your Windows Vista-operated PC an extra cache of memory to help increase speed, using the Windows ReadyBoost feature. Just plug the drive into a U.S.B. 2.0 port, and if it is fast enough to work with ReadyBoost, the system will prompt you to set it up. You do not have to erase any existing files on the drive and can indicate how much space you want to use for storage. The feature also works with Secure Digital cards. Windows ReadyBoost can use storage space on some removable media devices, such as USB flash drives, to speed up your computer.

Source: http://www.pctipsbox.com/increasing-the-speed-of-a-vista-operated-pc/