[H] curing system slowdown

[Veech]

I'm running XP Pro on a P4 3.0G system, 2G RAM.  The thing used to fly, and 
boot in about 30 - 40 seconds.  Now it's taking about twice as long to boot, 
in fact it takes a full two minutes from the time I hit the power button 
till the time the hourglass image disappears on the desktop.  That feels 
like forever.

I've loaded and unloaded a few things recently.  One program that may have 
been a problem was Half-Life 2 Episode One.  It required installing Steam, a 
nightmare in itself (in fact, I may quit the HL series because of this 
requirement).  I installed Nero 6 as well, and for some reason I can't run 
more than 3 or 4 operations on it (like copying complete discs from drive to 
drive) without it finally freezing up the system and I have to reboot it. 
Frustrating!

I did a defrag of the C drive, it helped just a bit.  AdAware and SpyBot 
come up clean.

So, what's a simple way to fix this?  Is it to reinstall XP?  Is there a 
website or program you guys can recommend that will help me diagnose this 
slowdown?  Apparently Vista will have an app just for this purpose, but is 
there anything available now that does the same thing?

thanks all..