I disagree with IO540's generalisation that the only way to effectively resolve issues is to reinstall a disk image. That is what professionals do because a) it is simple and b) they cannot spend the time doing othyerwise.
What I find is that I spend far longer than a paid technician could reasonably charge for. I do not charge and I do it purely for the fun of it.
For the others of us whom have 'real lives' and 'other things to do', copying off the useful stuff and reinstalling /re imaging/reformatting IS the best and most intelligent course of action.
Besides. Some may install a rootkit which is virtually undetectable.
Boot time virus checks may help, but add to your boot time, encouraging you to not reboot at all (which negates the value of boot checks)
You can EASILY become infected behind a router or firewall if you access the net at all.
Comodo Antivirus/Firewall is very effective even though the false alarm 'training' is somewhat annoying initially.