Paid anti-virus programs are not all they are cracked up to be, as long as you have one and an aniti trojan program there is not a lot you can do about it.

If you are realy worried about it there are a few things you can do and a few programs you can use.

Anti-virus
I like to use Avast Free Home i find it runs better on my system than AVG it has on-time scanning, background and e-mail scanning.

Anti-trojan
I use a-squared Free the free version is limited and has no background scan but it is still a fine tool to remove all the nasty trojansm bots, maleware and ad ware.

Cleaning

I use CleanUp this program is great to clean all the temp files, cookies and old junk you can do basic cleanups or custom cleanups and is safe for your pc unlike some others around.

Most of these programs have paid versions if you are interested in a bit more security but the free versions work well.

Changing passwords on a weekly basis is also a good idea use a password program to create your passwords so there are random.


There is another option you could start to use and this involves running all your web based stuff from CD rather than using windows nothing is saved to your hard disk you could even remove the hard disk and save to a usb key or usb drive if needed, this adds the best security as viruses and trojans can't stay on your pc as everything is in the memory and is erased on shut down.

http://en.wikipedia.org/wiki/Live_CD

Most of the systems are Linux based but are very easy to run and very much like windows.