This is a last resort, and it will cause you to loose all your files.
You may need to do a clean install. Do you have the XP cd's, or is Operating System on a hidden partition? Most olders Dells come with REAL XP cd,but most Compaqs,HP's,Gateways, and Emachines have a "hidden" partition that will let you do a system recovery that is on a 7-8 Gig Fat 32 partition.You can see it by click "start" and "My computer."
They also will usually allow you to make ONE copy (usually takes about 7-8 CD-R's) of the system recovery.,but for now you can just tap on the F10 keywhile its starting up and do a sysytem recovery from the partition.If you can get this to pull up click on "advanced options" and do a "destructive stystem recovery." This will re-format the hard drive and recover all drivers,applications.Just a warning all your data files,applications,and upgrades will be lost,so you'll have to re-install all that once your back online.
If it won't boot up by tapping on the F10 key, restart the computer and tap on the F8 key and select "safe Mode"and see if you can burn a set of CD's that will allow you to restore it.Click on start and "all programs" and look for "PC help and tools" and then look for a PC recovery cd-dvd creator. If you don't want to burn a set of CD's try using the (whatever brand) PC system recovery.This is different from the XP system restore that you've already tried.All that does is roll it back to the date you selected,but with this its going to wipe all the files from your hard drive,and re-install the OS like it was the day you bought it.If nothing else works this should take care of the problem,but you'll need to back up any important files.
If this doesn't help, maybe somebody a whole lot more computer savy than me will tell you how to get it up and running.