Why is my (Windows) computer slow? And what can I do about it?

Viruses, Worms, and so forth.

To Install anti virus software, run a virus scan.

Adware, malware, worms, etc.

Ok, so "malware" is not in the dictionaries yet, but it will be. Look it up using Google or find it at Webopedia.com. Malware is a general category of software that is harmful: viruses, worms, Trojan horses, spy programs, and adware. Adware could track your usage of the computer, or even initiate from your computer popup adds or program installation scripts of other programs.

These haven't been identified by the antivirus programs as "viruses" so you need to use one of the adware or spybot programs.

Install one of the adware scanners, then scan and remove these bad programs. Some programs may be identified as adware, but be installed by the user for use in a useful and legitimate way.

A Gazillion Temporary files

Maybe just a goggle. Every web site you go to has at least some temporary pieces (graphics or text) that your browser (Internet Explorer or Netscape) puts in a directory on your hard drive, usually the "Temporary Internet Files folder."

Too Many Programs trying to get to the Internet through a small "Pipe"

Windows Update, antivirus program(s), other data transfer programs, and your email program like Outlook or Eudora can all request data transfers when you first connect to the internet. If you have a small "pipe" (i.e., dial-up) and not broadband (DSL or cable modem), then these can easily keep the modem busy and slow down the computer until it is unusable.

Yes, you should disconnect and turn off your computer. But be prepared for the traffic jam. Broadband will help the transfers to occur quickly and let you get on to your work or fun.

Vulnerabilities

Windows update should be configured to run at least weekly. And other programs should be updated frequently.

Install and use a firewall.

Legal stuff:

As you should expect, this information is advisory and the actions described in general terms may not fit your specific situation. You should investigate further and carefully read the licenses for any program you install on your computer. Yep, you'll need a back up of your valuable files. And sometime you'll really need a backup...