Wiping a PC clean is always last result or in a case where my client wants it done. You can bring a computer back to good working order with numerous programs. Most common infection a person reinstalls a PC is a ROOTKIT, they are nasty and can be avoided. Most of the current ones are tamed such as Internet Security 2010, antivirus 2009, and a fake looking security center. I recommend always having malwarebytes installed (its free) run it weekly in combination with antivirus and you should be set. Be cautious when browsing the web and emailing, emails that may look legit from family and friends may be bad, such as a link stating to check out this video, when actually the link is an executable application it asks you to install. If you encounter anything similar stop everything and scan your computer. Most big name antivirus programs protect just as much as the other, only thing is how intrusive they are to you just like pesky Windows Vista.