Running an antivirus/spyware program
Sometimes the source of sluggish performance is the presence of a virus or malicious spyware. A good anti-virus program can purge your system of the bad stuff.
Always have a running/up to date anti-virus program running.
Windows Vista will constantly remind that your system is vulnerable. Windows XP is a little less courteous. It is rarely necessary to have all the additional features offered by anti-virus suppliers, most individuals simply need AV. AV programs (at least the good ones) should update themselves regularly. An attempt at a manual update will let you know if auto-updates are happening or not. (If it's already updated, then it's working.) Most will constantly scan your system, but, you should run a full system scan every three to six months at the minimum.
Run anti-spyware programs
There are free versions available on the Web, but do your homework and choose a good one. You can do this by checking the user reviews available at many download sites. Once you have an anti-spyware application installed, it is important that you run it at least one time per week. It is amazing how much "junk" is brute-forced into your machine by even the most casual Web surfing.