When you talk about "slow computer", what do you mean, exactly? Are you talking about a computer that was originally fast and became slow? Or was slow all along?
If this is a computer that was always slow, then yes, as "Sherry" said, you probably want to add more RAM.
However, if this is a computer that gradually became slow, there are things you might want to do:
1. Run your antivirus/ antispyware program, see if there isn't anything on your system that shouldn't be
2. You know how sometimes you install a program and all of a sudden, it adds a little icon to the bottom tray in Windows? Every time you turn on your computer, it starts running. If you have tons of those icons, this means you have a lot of programs that start running in the background as soon as you turn on your computer. You need to disable those *except* for your sound/graphics control panels and antivirus programs.
You do this by going to Start-->Run-->typing "msconfig"-->General Tab--> selecting "Selective Startup-->Startup Tap-->deselecting any programs you don't need to have running.
3. Lastly, check your hard drive for errors. This can make for a slow computer. You just right-click on the C drive, click on "properties", then click on "tools"
Hoped this helped!