My PC is running slowly....Part 2

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First part is at http://www.mylot.com/w/discussions/2361818.aspx I have done all of these things. I have AVG/Avast! I run CCleanr and YahooSpy everytime I shut down. I have checked the tast menu and very little is "running in the background" I don't have any software like games even on my hard drive. I even went and did a system restore to last months restore point before this all happened. And still...takes 15 minutes to boot up..and that is on a good day. Often, it just rolls and rolls inside that tower and I have to turn off all together cause it freezes up and won't let me click anything/anywhere. I have Windows XP...is there an Upgrade? I hve upgrades on automatic, and in my Add/Remove programs many are listed there. I use MOzilla Firefox as my browser not IE. I also have SP2... do I need to upgrade to SP3? I have checked my RAM and have 512mb, and 58k available to use, if I am understanding that right. I am not a computer techie so lot of things are foreign to me, but "version for dummies" is always a great help should you have anything to suggest to me.