|Dell 2400 series systems normally came with a Dell OEM XP disk and a Utility disk with the drivers.|
In order to use the XP Pro disk and not delete by accident you would need to be able to boot to the desktop and do an upgrade. From the symptoms you describe that is not an option.
You refer to System Restore, but that would need an OEM Home Premium disk. You would need to boot to the disk and in the first repair section trigger the system restore. I suspect that trying this will not accomplish anything. Do you see message on the screen prior to reboot?
What we need to find out is why you are getting shutdown when booting.
Will the system boot to SAFE mode?
Try booting and pressing the F8 key after the Dell bar starts loading. You need to be able to select the listing for "Do not reboot on errors". It sounds like a reboot loop caused by a BSOD problem.
You probably need to download a hard drive test program for the brand you have. If not the one I use that detects errors on most brands it the Hitachi Drive Fitness test program. Your reboot could be hardware based and this would help identify that.