Why would my computer still be slow, only three years old?

Note; I have a Compaq Presario - Windows XP SP2. I know they're the cheaper Hewlett-Packard, but this is getting ridiculous. I've always had the most recent version of Norton Anti-Virus and Internet Protection from the moment I booted the thing up from Best Buy. I'm constantly running Symnatec Live Update so it prepares my computer for new threats. I have it set so that it runs a full system virus scan every night at 3 in the morning.

I've done tons of research on what could be causing my computer this, besides the obvious virus / spyware possibilities. I've got a damn good disk defragment program that I pay for. I also have this scheduled to run every night around 3. I downloaded the legendary Ccleaner not too long ago to clean up some files and I got a good bit of memory from that.

It's not just internet speed either. It's slow opening documents and Microsoft Word etc. I have Comcast High Speed connection, anyways. I have a good 512 mb of RAM, and 116 gb of regular memory space.

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