Just to spell it out, here's a short list of "must have" programs that will help protect and repair your PC. Everyone has a different opinion about the specific brands of these types of programs. I lean very heavily towards freeware. The quality of some frewware utilities like these are amazing. My personal picks are the first in each list.
An anti-virus. Install only one - multiple installed programs can do very bad things. These will also scan all the incoming email messages automatically. Avast will not insert an advertizing blurb in each email like AVG does.
A good firewall. Anything is better than the built-in XP firewall ! Install only one.
An always-running utility that will detect when any program attempts to install an app that will be run automatically when your PC boots. This utility will show a pop-up that asks you if you want to allow that program to add itself to the system startup list. Very useful to prevent unneeded "bloatware" and malware trying to install itself without your approval.
Mike Lin's StartupMonitor http://www.mlin.net/StartupMonitor.shtml
Malware/adware installation blockers (both may be installed)
SpywareBlaster (*not* "Spyware Blaster") http://www.javacoolsoftware.com/downloads.html
Useful optional utilities to help maintain a fast, lean system. These must be run manually.
Startup Inspector http://www.windowsstartup.com/download.php
MS AutoRuns http://www.microsoft.com/technet/sysintern...s/AutoRuns.mspx
Malware detection. May not be able to disinfect. Must be run manually.
Ad-Aware SE Personal http://www.lavasoftusa.com/products/ad_aware_free.php
Spybot Search&Destroy Spybot Search & Destroy This is a dangerous program and requires in-depth knowlege for repair