Windows 7 introduced an option to use 'ReadyBoost' to help speed up a slow computer running Windows 7.
On previous versions of Windows, when RAM memory was low, hard disk space was converted into Virtual Memory; acting as temporary RAM. ReadyBoost means that the pagefile (virtual memory file) is moved to a chosen USB thumb-drive. Virtual memory on a USB is significantly faster than virtual memory on a hard drive thus speeding up the computer when virtual memory is needed. Generally, ReadyBoost supplements your computers RAM.
The following process is really only needed if your computer struggles to run Windows 7 efficiently; and is low on RAM. You may only notice differences if your computer has less than 1GB of RAM. However if you can run windows fine, and have spare USB drives lying around, then I don't see the harm in doing so anyway- it may come in useful.
It should also be noted that this process can run-down a USB, as it may be accessed often. Because of this, ReadyBoost should be seen as a temporary fix for a slow computer.
1)Insert Flash Drive (the bigger and faster the write speeds, the better)
2)An auto-play window should pop-up, asking what to do with USB
3)Scroll down, and click 'Speed up my System with Windows ReadyBoost'
4)Choose whether to dedicate the whole USB to ReadyBoost, or only part of it by sliding the slider
5)Click 'Apply', and then 'OK'