Over time, by installing/uninstalling programs, surfing the net, downloading from the net, not properly preventing and/or detecting and removing spyware, malware, trojans, viruses, etc., and not keeping your temp files, registry, etc. clean, things can get messy with your operating system, and performance will suffer. As you can see, there can be multiple reasons for your loss of performance. If the performance loss has been reasonably fast, and dramatic, Nodsu's suggestion is the best way to start. Having just Microsoft AntiSpyware and Norton AntiVirus is not sufficient. You didn't mention if you have a Firewall, but I'll assume you do. I suggest you research the following:
1) Freeware programs which help prevent infection, like SpyWare Guard, Spyware Blaster, Win Patrol, Spybot Search & Destroy, etc..
2) Freeware programs which detect and remove infections, like Ad-Aware Personal SE, Spybot Search & Destroy, A2, CWShredder, HijackThis, F-Secure Blacklight, etc..
3) Freeware cleaner programs which clean up the Registry and System Internals which are faulty, and clean out the temp files, like CCleaner, Spybot Search & Destroy, etc..
In addition, you should defrag you HDD(s) if you haven't already done so, and check and adjust the size of your pagefile to make sure you are not over-utilizing your RAM because your pagefile is too small relative to the amount of RAM you have.