Not that any flavor of Windows is speedy, but if you notice your machine running slower than usual, it might be the wuauclt bug in Windows. How to tell?
Run task manager and look at the processes. If you see process SVChost.exe using 99% of your CPU time, you have the problem. You are looking at service wuauclt.exe locked up in a loop. Microsoft does something unneeded and unusual here, they run one program (wuauclt) but make task manager display the name svchost, rather than the program's true name of wuauclt. This is pure obstructionism on Micro$oft's part.
You can regain control of your machine by using task manager to kill process svchost.exe. You can ignore the scary warning task manager displays when you do. Nothing bad will happen. Go ahead and kill it.
Various internet postings tell me that wuauclt is part of windows update. Windows update is supposed to go thru the list of patches for various things, and sort thru the patches that are new, and the patches that have been already applied, and just apply the new patches. M$ programmers bungled this bit of code, and as the patch lists get longer and longer, wuauclt gets into a loop and hogs 100% CPU time for as long as a hour.
If like me, you are running trusty old XP, you might as well turn windows update off, M$ isn't issuing patches for XP anymore. You can do this with services.msc, which you can start from the run box. Find "automatic updates". Stop it. Then click properties and set the startup mode to disabled.
According to a couple of internet postings, the wuauclt runaway is caused by or aggravated by missing patches in Internet Exploder. There are multiple versions of IE out in the wild, V6, V7, and V8. You ought to update to at least V8. And then run Windows Update (if functional) from within IE to apply all the outstanding IE patches. I did that, haven't run long enough to say if it works. But it's a good thing to do in any case. Better is to use another browser, any other browser is faster and safer than IE. I only keep IE on my machine to run Windows Update, I do all my web cruising with Firefox.