My computer is inexplicably slow and seems to just stop for no apparent reason. What can I do about it?

I am running Windows XP Service Pack 3 on an HP Pavilion PC with a 2.13 GHz CPU and 2 GB of RAM (now past its warranty period). I have DSL which I leave connected to the Internet most of the time. It too seems very, very slow.

My computer seems to just space out or slow to a crawl. I have tried uninstalling a lot of unnecessary stuff, which helped a little bit. I have System Mechanic Pro which I use to diagnose and fix things like registry errors, fragmented files, Internet clutter, and all the usual suspects of slow performance.

When I look at Windows Task Manager, the CPU doesn’t seem to be particularly active during these “take a dump” sessions; in fact, it is usually around 10% or less.

Rebooting doesn’t seem to help much, and I try to avoid it because that is very s-l-o-w—i.e., on the order of 20 minutes to shut down and power back up to where I was.

Sometimes it takes several seconds for the menu to come up when I right-click; other times it comes up instantly.