After you reinstall your windows xp or vista, you need good software.
If I were you, I would buy one program and use for the rest you need freeware. The program I advise you to buy is
TuneUp Utilities 2008. It has 15 different programs
http://www.softpedia.com/get/Tweak/System-Tweak/TuneUp-Utilities.shtml
TuneUp 1 click-maintenance
TuneUP Disk Doctor
TuneUp Disk Space Explorer
TuneUp Drive Defrag
TuneUp Registry Editor
TuneUp Repair Wizard
TuneUp Rescue Center
TuneUp Shredder
TuneUp Strart Up Manager
TuneUp Styler 2
TuneUp System Control
Tuneup System Info
TuneUp System Optimizer
TuneUp Undelete
TuneUp Uninstall Manger
The good thing is, that the 30 days trial are without any limitations, so you can try out everery program
When you had to buy these programs seperately it would cost you hundreds of dollars
For anti virus I would install
Comodo anti virus
Comodo anti malware
Comodo Firewall Pro
http://www.comodo.com/
Anti Spyware
Spyware Terminator
http://www.spywareterminator.com/
Registry and cleaner no need when you install TuneUp
But just in case
http://www.ccleaner.com/
http://www.freewarefiles.com/Glary-Utilities_program_34666.html
Media player
Gomplayer. It is free and can play any file type and no need to download and install extra codecs etc.
http://www.gomplayer.com/whatisgom.html
For playback of music WinAmp. The best there is
http://www.winamp.com/