If you got a computer within the last 3 years, most likely it have either Window Vista or Window 7. Compare to the previous version of Windows XP, these two versions have a feature not too often use but a quick, easy, and cheapest way to speed up your computer. It is called ReadyBoost.
All you need to do is get a USB thumb drive. Make sure it is blank otherwise the files would be erased. Plug the thumb drive to a USB port and Windows will ask if you want to use the Ready Boost feature. Just select yes and you are done.
What is the advantage of using Ready Boost? By default Windows use space on your hard drive when it needs extra memory when large programs such as Photoshop, games, and even surfing the web called Virtual Memory. The USB flash memory is about 10 times faster than the hard drive therefore making your computer faster.
I was able to pick up two 4GB flash drives for $5 and two 8GB flash drives for $12 at a local Office Depot last month. These drives are relatively cheap which makes a quick and inexpensive upgrade. Even though if you use an 8GB drive, the ReadyBoost feature can only use 4GB, so the other 4GB can be use as a backup.
If you want to know more about the ReadyBoost feature, there is a great article at ZDNET that gives more detail.