Pr0n and such are NOT the only source of viruses. These days, a lot of major web sites are from time to time infected with so-called "drive by" viruses - embedded in altered java script and other web pages. My approach is this:
Use a good virus scanner.
Use a web browser other than Internet Explorer, such as Mozilla (Firefox) or Chrome.
Disable java script and java applets/plug-ins for the browser, or at least require your permission to run them.
Don't visit web sites you don't trust.
Review your email. Anything with a link should be checked with your A/V scanner.
With a clean system, make regular system image backups to an external drive of the ENTIRE disc - some viruses will infect the boot loader and files in your recovery partition. Use a live Linux CD or DVD to do this. I can provide detailed instructions how to do that if you wish.
Scan the system on a regular basis.
After scanning, backup your data files to an external drive.
Use Linux - it is a lot harder to infect than Windows (just my opinion, and a shameless plug for Linux).
Make sure that the external backup drive is kept detached from the system except when either backing stuff up, or restoring the system.
If you do get infected, don't just attach the backup drive to restore the system. Do this:
Boot with the live Linux CD/DVD that you used to backup the system drive.
Restore the backup image to the disc.
Reboot the system with the restored image (detaching the backup disc).
Restore the backed up data files.
Rescan the system.
Is this a PITA? Heck yes! Is it necessary? Absolutely! You either stay at risk of losing everying, or you become smart about preserving the integrity of your system. The choice is yours. Do it, or lose it...