I think it has reached that time that the only sure fire way to get this resolved is to do a Full Reinstall. It's always my last resort, but occasionally that's what it requires. If you would like me to, I can always go through the process of reinstalling Windows on your PC, just let me know if you need any assistance
The steps for performing a full reinstall can be found here.
Summary of what you need to do when reinstalling:
Backup any data that you want to save onto external media such as DVD's, USB Sticks, External HDD's etc
Reinstall Windows (using the disc)
Install all the drivers (so your Sound, Ethernet port, Graphics etc work as they should)
Download any remaining Windows Updates
Copy your data back over to the Hard Drive and reinstall any programs that you want back on
Another guide on reinstalling XP is found here and that contains good details on the steps and shows you screenshots as well. You would just need to read from the Steps to Clean Install XP section on that webpage. Most of the screenshots there will be very similar to the ones you will see when running through the Dell CD.
Remember though, backup your data before reinstalling as they will all be removed as part of the reinstallation. Most people will backup items such as Pictures, Documents, Music, Favourites, Emails etc. You can't backup programs though, as the files associated with the programs are installed in a number of different places on the PC, including the registry. So programs such as Digital Camera software, printer software, Office programs etc will need to be installed from a disc or downloaded, once XP has been reinstalled.
Let me know how you get on or if you have any questions.