slow functions on desk top computer

I used to work in a secondary school repairing PCs, if asked to fix a "slow" one I'd generally check the following:

RAM (we had 1GB as a benchmark for XP SP3, but generally the users weren't running photo manipulation software so you might want more)

Hard disk fragmentation - if your hard disk seems to be churning away a lot while loading programs, check this. Just right click on "My Computer", select Manage my computer from the menu, and then find disk defragmenter in the box that appears. You may find that you need to move some files in order to allow this to run properly as it requires a certain quantity of free space (15% from memory?) but it should help matters.

Check for spyware too - there are free programs such as Spybot Search and Destroy or Adaware that will remove most forms.

Do not, whatever you do, install anything promising to "optimise your PC". Most of these programs are spyware or nagware that will make the problem worse, the same goes for web popups shouting at you about a virus. What you install is in fact a virus which manifests itself by constantly trying to scare you into paying money. Avast! antivirus is genuinely free for home users and does a great job.

We had one case of a teacher managing to install one of the fake antivirus programs. It was quicker to wipe the hard disk and reinstall a clean image from the server than it would have been to try to clean it out manually!