Perhaps its time for a cleaning.  XP does not just magically become slow (ok, it sort of does, but that takes a lot of time... like a lot).
Make sure you are completely up to date, if not, then I suggest backing up your files, wiping the system and starting over.
For example, if you are running Norton <insert product name here> 2007 for example, and SP1 then you need to back up and reformat STAT (short for statim: latin for immediately, cool huh?).  Then, once you have reformated, install your drivers, and run updates until you have SP3.  Whilst doing that, install Avast, AVG OR (not and mind you, only 1 AV program) Avira.
Make sure that Windows is set to update automatically as is your AV.
Now, if all that is already done, in the sense that you are running an up to date AV and Windows is up to date, the next questions are these.
When was the last time you defragmented?
How much RAM is in the computer?
What speed is the CPU (and what kind).