I've known about Sysinternals (now owned by MS) for a long time, but I didn't, and still don't have a full appreciation for the extreme value of their utilities.
For a long while now, I've been trying to revive Qian's computer. It's an old IBM thinkpad R series, with a 1.13Ghz P3, and 384 megs of RAM. And it's dog slow. With more than one app open, it starts thrashing. I poked around with task manager and even Process Explorer to see what's going on, but things seem pretty kosher.
Today, I tried the Autoruns utility as well as the PageDefrag utility, both from sysinternals. The former gives you a huge catalog of all the things that can automatically get loaded in your system. I disable a bunch of unnecessary startup items (qttask, and webcam related items). I also found that the "Logitech Desktop Manager" app had registered a bazillion protocol handlers (literally like 50 dlls) which I think were being loaded in to explorer.
The PageDefrag utility shows you the fragmentation state of the two big system files on your computer: the virtual memory swap file, and the hibernation file. In this case the swap file had only 5 fragments, which isn't cause for too much concern, but the hibernation file had 32,000+ fragments!
Because the system takes exclusive locks on these files, the PageDefrag utility gives you an option to defrag these files at next startup. As I write this, it's been working on the hibernation file for a good 5 minutes.
Of course, I know that the only real way to make this machine faster is upgrading its hardware -- maybe 700MB ram and at least a 5400rpm drive. But at least now I have a hope of getting its performance back to the fresh OS install level.