first things first:
1. What OS do you use? If you have windows XP download all updates and service pack two. this will help a lot
2. Do you use programs such as Kazaa? Weather Bug? Comet cursor? If so, GET RID OF THEM. These programs infest your computer with memory hogging spyware.
3. How do you access the internet? Is it through high speed or dial up? If it'* highspeed make sure you go to "my network places" right click the connection, and enable the firewall. If you have Service Pack 2, make sure you have the firewall enabled.
4. (don't have to answer this one) but if you go to shady websites, (such as ****) you could have spyware from there, even viruses.
5. You need a good virus scanner/realtime scanner, in addition to spyware. A virus can make the same symptoms you are describing.
6. download adaware. It seems to be a great program. Make sure to run updates on it atleast once a week and scan for spyware often.
7. Run liveupdate on your anti-virus software. (I do it atleast once a day. You can set it up to automatically run.)
8. Ditch internet explorer. IT SUCKS. use a broser such as Mozilla firefox or netscape. They're more secure, plus you're much less likely to get third party programs installed on your computer because of them. Tons of stuff gets through IE through flaws.
By following all these steps I have never had a computer virus on my computers.. they're running in tip top shape even after all this time.