I used to use Avast - and still do on 98 machines. There are issues about its performance with XP.

Of the free antivirus programs I like Antivir. AVG is causeing people problems these days. Paid antivirus programs ALL (except one) seem to cause problems sending email. Even with the recommended workrounds in place, Outlook disconnects from the internet after it has sent the email to the antivirus, and befire the antivirus has sent it to the internet. The exception is Panda antivirus.

Antivir needs a separate software firewall. I try to avoid Zonealarm because it slows systems down SOO much. Not all firewalls work with all OSs. Tiny is quite light on resources if you can get it going.

But the main purpose of this post is to sing the praises of Norton Ghost. It came free (version 7) with my motherboard. Create a aeparate partition before a format C - call it, say, E.   It's going to be small, less than 2gig (98) because all that goes in it are the programs.   Make another one called F. Format C, clean install. Now you have a system with no crap in it. Boot to DOS. Run ghost from DOS - follow the instructions and put a copy of that clean system in E.

Go to the knowledgebase and follow the instructions to put "My documents" on the F drive. Same for your email client files (in my case outlook). PUt the folders for your other data (pictures, music, new programs downloaded music, etc) in "My documents". NOw all your data goes to the F drive.

When your system goes tits up - Boot to DOS and run Ghost. Put the contents of the E drive back over the top of the C drive. It's that simple. This restore takes 15 minutes, and you have a fresh clean install and are running again. I actually did it on Friday. My last clean format C and reinstall from scratch took me 4 hours, and I was lucky to be able to save my docs and psts off to CD first.

For more info go to the Radified guide to NOrton Ghost.