People bring me infected computers all the time. I keep an old XP computer clean and loaded with fresh scanning software. I pull the drive from the infected computer and attach it to my computer via USB and run multiple scans to clean it as best as I can. I then return the drive to its original computer, boot it in safe mode and run ccleaner to remove as many temp files as possible, update the AV software and then run a few scans. Some of the free products I use are Clam WIN portable (portable=no install), Norman anti malware, super antispyware. Sometimes I have to also enlist Hijack this, combofix & a few others.
Be very careful where you get your tools. There are bogus versions of many popular products or ones with very similar names. I trust filehippo.com for many utilities.
Keep your systems up to date as far as patches, windows, Acrobat, Java, flash and other common software we all use. Most infections come from unpatched vulnerabilities. Consider using alternate software like Foxit reader instead of Adobe Reader or Firefox instead of Internet Explorer. Hackers target the most used software.
Learn what to do when an infection presents itself. These rogue AV infections can come from advertising placed on a legitimate sites by a 3rd parties. They generally employ social engineering. You get a popup with a rapidly changing screen you you feel compelled to hit something fast like 'install this yes or no'. Hitting No is no guarantee it means No so the best thing to do is load task manager (Ctrl,Alt,Delete) and force your browser closed. Do not select ANY option presented to you by this bogus popup. Run ccleaner and then do a full scan to be safe.
Cruising warez (pirated software), pirated movie or music or porn sites is an invitation to be infected. If you have a need to, use a computer where you boot of a live CD of linux to be safe and make sure your internal hard drive is not 'mounted'.
Be suspicious of any email attachment you receive, even from people you know. Last week I got an email from Amazon about an order I placed. It came to an email address (not gmail) I don't normally give out. I scanned the attachment with Security Essentials and it appeared to be clean. I sent it to Microsoft for further analysis and they reported it as dirty and updated their definitions.
I use gmail for most of my email addresses because they do a great job in filtering out spam, bogus emails and infected files.
My 2 cents.