Computer is hanging when loading up the desktop

Hi guys, before I say anything else, it's worth noting the system in question Dimension 2400.

Well, where to start? The computer was having a lot of trouble with IDE devices, I decided to swap the motherboard and after trying everything I could think of, and swapping pretty much every part possible, it still wasn't booting up correctly so I decided to switch back to the old board.

The system was hanging rather often, and it just wouldn't pick up either of the CD drives, after much messing around, both the BIOS and Windows are picking up both CD drives just fine now, but it's still hanging. Basically, when you first login it'll take maybe two minutes to even begin to load up the desktop and all of the icons, and once that's finished it'll take another 3-4 minutes to finish loading a few extra programs (such as MSN Messenger for example) and actually give you access to things such as My Computer. After these initial 3-4 minutes it runs almost completely smoothly until you try to log off, where it hangs again and generally takes ages.

I have tried doing a Windows repair but can't because it has XP Pro installed but the CD I have is XP home and am currently running AVG but it hasn't found anything as yet. I think a lot of these problems came about when she tried installing her new broadband/wireless router, I've just noticed the firewall is disabled and when trying to re-enable it I'm presented with this error - http://img264.imageshack.us/img264/8067/925071245dt.jpg - could it be related?

Basically, I think the computer is in need of a complete re-install, but I've absolutely no idea how to go about it since it's a Dell. I've actually heard reports that Dell BIOSes are set up in such a way that it absolutely will not let you boot from a standard Windows XP CD, and you are required to have a Dell recovery CD which is something I don't have; is it possible to download these in an ISO format or similar? I've already got the key on the side of the case so that won't be a problem.

Any suggestions on what course of action to take please? Should I at least try to format and hope I can source the relevant drivers from somewhere? Is it possible to maybe download an ISO of the recovery disc appropriate to the machine I'm trying to fix (Dimension 2400). And help would be very gratefully received, I'm doing this for a friend and have had her computer for well over a week now trying to fix it, I bet she's getting really annoyed with me now, hehe. Thanks!