Antivirus:

AVG Antivirus - FREE, Works just as good as all the rest you pay for!
Windows One Care - Virus & Spyware protection, PC-Tuneup, Backup Files

Spyware Defense:

Free Spy Audit. Scan your PC for spyware 

Windows Defender - FREE active spyware protection for Windows XP, 2000
Windows One Care - Virus, Spyware protection, PC-Tuneup, Backup Files
AdAware - FREE Spyware scan and remover
Counter Spy - finds hard to reach spyware
Webroot Spy Sweeper - another additional program for bad spyware problems
Spyware Doctor - a newcomer, a good alternative.

PC Tuneup Software:

Auslogics FREE Disk Defrag Software
System Mechanic 6
Win-Patrol - Stop unwanted startup programs
CCleaner - safely clean unnecessary files and registry
Winsock Fix - Reset winsock files and hosts files to repair unknown problems with Internet Cnnections.

Keep Internet Explorer Maximized -

Download the Gmail POP Troubleshooter.