Machines can run slow for all kinds of reasons and, as Softus suggests, it can take a while to resolve. Most of the machines I see either have bad housekeeping procedures, spyware, leaky applications or duff hard disks. Occasionally, a bad stick of RAM can do this, but more often than not, it's disk/data related.
I usually take this approach.
1) Check RAM installed. IMHO, XP requires 512MB RAM to run comfortably with basic Apps, like Office etc. I would always add this as an advisory if it were any less. Also, check that you are not running out of disk space, especially on the drive holding the page file.
2) Check processes and the amount of background CPU being used. You can do this from task manager as suggested.
3) Identify the hard disk, download and run the appropriate diagnostic tool from the manufacturer site. If this reports a failing drive, then forget the rest of the tests and salvage what you can now to a new drive.
4) Run a disk cleanup to remove/empty temp Internet files, recycle bin, temp files, old system restores etc.
5) Run a scandisk to check for any basic anomalies. I usually boot from the XP CD and run it from the recovery console, but you can schedule it from Windows.
6) Run a spyware and virus scan using Spybot/Avast or your preferred programs.
7) Run a fragmentation analysis and defragment if necessary. I usually disable the page file before running this, and then enable it again after. There are tools that will sort out the page file on-the-fly.
If all the tests pass and the above procedures have been followed, there should be an improvement if it is a disk/data problem.
