There are a lot of quicks tweaks to your computer to speed it up. This is especially true if you have a computer that's been used for awhile – particularly donated ones – when you don't wish to reformat and reinstall the operating system; however, even with new units you'll want to disable some items that automatically run at startup.
In particular, first I'd go to START > PROGRAMS > START UP and delete anything I don't need to have running (which is anything besides my calendar program to remind me of appointments). I then would go to START > RUN and type in “msconfig.” Then i pick the “selective startup” tab and begin to uncheck anything that doesn't look like it needs to automatically start. Examples are anything to do with realplayer, hpscheduler, AIM, wildtangent, etc. It's important to realize that you can go back and “re-check” the items you unchecked—so you don't need to worry about killing your computer! Leave the following items checked: Scan Registry, Task Monitor, System Tray, Load Power Profile, Virus Scanner and anything related to your anti virus. You'll need those to be there all the time. (As a a matter of fact, I just went through a friend's sluggish computer yesterday and used an anti-spyware program, tweaked the selective startup settings, and deleted lots of nasty looking programs that she never used—as well as installing Firefox—and now it's running great.)
For more information, go to www.sysinfo.org/startuplist.php – this is a great little webpage someone put together regarding speeding up your computer. Also, USA Today published a nice article entitled ”Five steps to help speed up your PC” .