It would sure help us if you told us more about the computer such as operating system, when was the last time you took the cover off and cleaned out the dust using compressed air? Will it start in Safe Mode? etc. The more information you give us, the better informed our answer will be.
On an old machine like that, assuming it won't start in Safe Mode or "normal" Windows is the hard drive.. It sounds like you get a display which shows the Dell flash startup display and as such, you're probably able to access the BIOS/CMOS settings. Is that correct? If that's the case, then I recommend replacing the hard drive and reinstalling everything back to it's factory state using the Recovery Discs that came with your computer.. Once that's done, you can reload all the personal files and documents you've saved on your backup discs.
Hope this helps.
my dell dimension 4400 is about 6 years old (i know, ancient) and all i get when i turned on is the dell world screen. i've tried restarting, but nothing changes. nothing works. any ideas?