OT: Windows Optimization?

[AgentZero]

I've got a 2.4GHz P4 with 512MB RAM and about 6GB free hard drive space running WinXP SP2, but for the Last few weeks, the system has been running really sluggishly. Games seem to be more or less unaffected, aside from noticably increased loading times, but when I'm working on the desktop, lots of simple things seem to take a long time.
For example, when Windows loads when I turn on the PC, it takes 4-5 minutes before I can actually DO anything. The desktop is there, everything seems to be fine, when I click an icon it highlights, but if I double click to open a folder or run a program, nothing happens til I've left the machine alone for a while. Even then, sometimes just opening a folder takes 10-15 seconds.
I've got about 3-4 anti-virus programs and they all say the system is clean. I've downloaded a few of those Windows optimization programs but they haven't been much help.

So, I'm wondering if this sounds familiar to anyone, or if anyone has any ideas as to what I can do to get the performance I think I should be getting. I've tried everything I can think of and nothing seems to work.