Do your operating system updates, start - windows update if thats what you are running.
Look on this site for spyware removal tool links and do those things (most are free). adaware, spybot, and others are additions to anti-virus that you need. Norton by itself doesn't do all you need. malware is rampant and causes slowdowns because it uses your computer.
If after doing all the checks you still have a slowdown, then start looking for what may have been added or changed while you were gone (besides malware).
Past that look for emerging hardware problems or heat issues. Try running the system with the case removed to see if cooling will help. Make sure the inside is clean and dust free (be very carefull & cautious - no power).
As you eliminate things, you can post more specific questions and include your system type, O/S and full configuration for better help.
I have had a slow comp lately. I have 504 MB RAM persario and the comp is very very slow. Also some programs will not load fully. I have just recently noticed this problem as i just got back from a job training trip. My kids were on and they said they noticed it. I have tried almost every Norton Removal tool and have had no success. I have done all the usual stuff(defrag ram, defrag drive, cleanup, etc.) and wanted to know if anyone has any input on if this is a worm, virus, trojan, etc. or if it is a overlying comp problem. Also I have looked into memory leakage but even when i dont run programs it is slow. Any and all help is appreciated
-steve

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