Friends do NOT let friends install Norton Antivirus. I'd stay away from MacAffe as well. Both of these two products have caused more problems for me (as the family IT guy) then any virus/malware/worm ever has.
Best alternative is
AVG Free.
For spyware/malware I use the Microsoft Tool +
AdAware.
If you know what software is installed on your system, check out the
threat explorer at Symantec. They often have targeted removers that deal specifically with the virus/malware that is on your system.
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And I'm sure it's been beaten to death, but if you're still using IE6 or below, upgrade to IE7 or use a different browser.