An interesting and useful blog at the Guardian is 'Ask Jack'. Recently he dealt with a question about a slow desktop machine. Some might find it useful.
http://www.guardian.co.uk/technology/askjack/2010/oct/06/slow-windows-pcA PC running Microsoft Windows XP should continue to run at the speed it did when you bought it.
They never (or very rarely) do in real life, for a number of reasons. The most obvious is that both the operating system and the applications change as security holes are patched and new features are added. Today's updated XP SP3 needs more resources than the original version, launched in 2001. To run Windows XP SP3, a browser and one main program nowadays you should have at least 1GB of memory and at least 1GB of free hard drive space.
In the article he mentions AnVir Task Manager Free. I downloaded this and it is a pretty good tool for tweaking a lot of settings - with one proviso; there are a couple of links in the free version that are advertising for online services such as online registry checker, driver update etc. I would strongly advise AGAINST clicking on these.