AVG is all you could dream for;
Free off the net, only an email registration and you get a serial no. which lets you install it on your computer.
The virus scan is updated automatically every day by connecting to a server, so free updation is not an issue.
The virus scan has detected trojans and worms that non of the other virus scans could detect ( like norton, mc affe, PC-Cillin, and innoculate to a certain extent).
AVG even has an emial scanner where every mail you receive is sent to you after scanning from https://grisoft.com ( the software is available free here).
But beware, this is a memory resident software and it can reduce your RAM effectiveness. Also when initially loading AVG, if your computer has a lot of viruses, it may require a reloading of Windows. So be careful when loading it and make sure you have a Windows CD next to you.