When you reboot your computer, about the time you see the C: prompt, start pressing the F8 key (oops, F5 - it's a Dell) numerous times. If you started soon enough, the computer will bring up a start-up "options" screen and wait for your input. Press whatever arrow or key is necessary to tell it to start in the SAFE mode, which will bypass most of your normal programs but should allow the computer to start. You might be able to find the problem from that point, if you understand directory and file structure.
Do you have an active AntiVirus program? (I hate Norton, McAfee is okay but I really like The Shield Deluxe) If so, get to it and run it, to completely check your computer. If it needs a couple hours to do that, let it. Most of the antiviral websites have the ability to scan your machine online, usually for free.

If you cannot get into the SAFE bootup mode, I hope you have a good antiviral program AND made an Emergency Startup Disc, because that is your next option. Find that Emergency Startup Disc and boot up your machine with it in the floppy drive (or CD drive if that's the kind you have.) Run the diagnostics and cure the problem.

After all that, you may need to do a REPAIR of your WinXP installation if the machine will not boot up properly still. Boot up your machine with the XP installation disc in the drive and tell it to repair your installation.

The WinXP disc also has a Repair function on it that might work. Just boot to the disc and you should see that option. If "Repair" doesn't work, you may need to completely re-install the Operating System (WinXP) which will force you to reinstall all your other programs also. This should be considered as a last resort.

If that doesn't fix it, you are hosed. I would NOT suggest you transfer ANY files to your new machine unless and until you have an Active AntiViral program running on the new machine. That includes the use of any floppy disc or any re-writable CD disc, as they may already be infected (if it's a virus).