Spyware Doctor is great. Norton however is awful, AWFUL software.
Seriously, remove it and your PC *will* speed up. It might or might not be the cause of your current situation, but it will stop your PC from running like a one legged dog. Don't just disable it, uninstall it, and burn the CDs
A wonderful and *free* antivirus program is AVG Anti-Virus -
http://free.grisoft.com/ - which is fantastic, stable and regularly updated.