If your computer is driving you crazy because it's too slow, and your budget is tight, consider getting more RAM to see a drastic improvement in your system. If your computer is slow, and you hear your hard drive working over-time, its likely that you could see a significant improvement by just adding around $50 worth of RAM. The first step is to see how much RAM you currently have: Go to "Start" / "My Computer" / "View System Information". On the "General" Tab, you'll see a summary of your general system information. If you have less than 1G of RAM, you will definitely benefit from more RAM.
Yesterday's Boston Globe had a good article outlining a bunch of measures to revitalize your old computer: http://www.boston.com/business/technology/articles/2008/10/23/stretch_your_money_by_stretching_your_pcs_life_span/
Don't be disappointed if you can't afford a new computer right now, because there is a good chance you don't need one. In our experience, adding RAM is the highest-impact, lowest cost way to boost performance. If your computer can support 2G of RAM, you will see a BIG difference
Note that there are many different types of RAM, and there are specifications for the amount of RAM that can be added based on specific system models. If you want to go the "do it yourself" route, we recommend going to the manufacturer's web site (Dell, HP, etc) and entering your system information, or use the "RAM Finder" function that all of the major RAM manufactures have, such as at http://www.crucial.com/.