Computer is annoyingly slow

I am trying to help someone with a computer problem and could use some

This person says their computer became very slow after they finished
playing a computer game. I cannot figure out why it's so slow.

It's a new (3-4 month old) custom built computer running XP Home. It
has an Intel Core (2-?) Duo processor, 2GB RAM, and a 150GB hard
drive. They use AOL and Norton Internet Security 2005. I did two virus
scans using Norton Anti-virus and the Trend Micro Online scanner and
nothing was found. I did two spyware scans using AOL's spyware scanner
and an online spyware scanner. Some MyWebSearch toolbars were found
and I removed them. Using HijackThis I found a file called ofb1.dll
that is suppposedly a trojan, so I removed/fixed it. I also did a
rootkit scan and nothing was found.

I have checked for conflicts in the Device Manager and everything is
ok. I have disabled Startup items. In the task manager, everything
looks normal. The cpu usage is 2%, System Idle is 99, the memory usage
is low and doesn't look unusual.

I cannot think of anything else that would be causing this slowness.
Now when I say so I mean slow. It takes 30 min to open a folder and
then open an application. It takes over 10 minutes for all the desktop
items to load on a reboot. The only improvement I've seen is that now
when I open task manager, it shows up immediately and all the tabs are
now there. It wasn't coming up a few days ago. I ran defrag and also
chkdsk and no errors were found.

I cannot think of what else could be wrong. Does anyone have any

I'd prefer to figure out what is wrong before doing a System Restore.