Begohan

My computer keeps freezing for no reason at all. It starts up normally, and everything loads just fine. I go to open MSN for example, it opens, but I try to do anything inside of that program like open up a new window and it freezes. I press end now and it doesn't close no matter what, even after hours. I minimize it, go to desktop, try to open a folder and explorer freezes. At this point nothing opens up nothing works and I restart. I have restarted plenty and its the same story every time. I hadn't done anything different than I usually do with my computer. Just one morning I go to my computer and its been doing this non-stop ever since.

I have Windows XP Media Center SP2. When I start the computer in safe mode, this problem doesn't occur so its obviously software, but what could be causing this? I have Nod32, Ad-Aware Pro 2007, and Comodo Firewall which are all realtime protection and they scan every night and update every hour so I'm highly doubting I have a virus and they're not causing it because its been like that for 5 months.

Its a year and half old computer, and I'm fairly advanced with computers so I know what I'm saying is accurate. No new hardware or software in particular has been added to my computer recently. It is a Dell Dimension E520 computer. I ran chkdsk /p and /r, and it never solved anything.

Please someone help me, I need my computer back ASAP.