XP can sometimes be a very buggy and bitchy operating system; it has several problems with 'irreversible' option changes and can be very annoying to keep maintained.
Many of these tips and programs also work on Vista and Windows 7, just check the fine print to be sure.
So.

Here is some useful tips for keeping XP, running clean.

Jerms guide to PC Repair and Maitenance

Firstly- repair.
If something goes wrong, you're lost for answers.
www.kellys-korner-xp.com/xp_tweaks.htm
This site has many excellent registry fixes as well as many tutorials on fixing common XP problems.
It has a "tweaks" section, this allows users to edit XP in ways that suit them.
HOWEVER.
Be careful with these sort of edits and fixes, they are; registry edits.
The registry is a key part of the Windows operating system, and fussing about with it when you are unsure is a very foolish thing to do.
en.wikipedia.org/wiki/Windows_Registry
Understand what it is you are doing, before you do it.

Secondly- Maintenance.
XP has several tools to maintain your PC. Defrag, disk cleanup and Uninstall utilities.
But these aren't the only ways to keep XP in top shape. If you want to ensure your PC is tidy and clean, try these small programs-
www.revouninstaller.com/ - The ultimate uninstaller tool, finds the small programs that you never knew about and also removes old files and shortcuts from your uninstalled programs. It also has a Junk Cleaner (more on this later) and a tool to manage what programs start up when you boot your PC.
www.piriform.com/- Two programs are on this website, Ccleaner, and Defraggler.
Ccleaner- This happens to be an amazing tool. It is basically a "Crap Cleaner" (hence the name). It removes temp files as well as dead shortcuts and other useless stuff that takes up space. This program will most likley clear up 500MB + on it's first operation, and continue to clear such amounts on each use in the future. There is a smaller version of this on Revouninstaller, it clears up alot more and has a "registry cleaner" which clears up the unused links in registry (BE SURE TO BACK UP YOUR REGISTRY! It prompts you before hand!)
Degraggler- The ultimate defragging program; quick, effective and most importantly, accurate. It will not leave that "2%" left fragmented. Very useful, and much shinier than the Windows defragger.

If all is lost-
www.bleepingcomputer.com/
This is the site where the computer technicians go if they are lost for answers.
It is a simple forum site, but it's community is more powerful than any phone hotline.
Simply make an account, and submit a topic in the appropriate forum stating your problem and question, be precise on what is wrong and be polite.
It is also a great place to go if you want to do something on your PC, but am unsure how.
Responses are usually well written and thorough.
If your question/problem is not answered in 2 nights, post the question again.
By the time you post it 4 times, your problem will DEFIANTLY be answered.
It just depends on who sees it.
However, your simple problems should be answered in an hour or so.


So.
I hope that helps!