Are you very sure you cannot start your computer in Safe Mode? If you
can boot into 'Safe Mode with Command Prompt' you can use System Restore.
If you can't boot into Safe Mode things are a lot more complicated. You
can boot to the Recovery Console from your XP install CD, but only if
you have an XP install CD and that CD is at the same service pack as
your XP installation. If you can get to the Recovery Console I would
show the computer to a professional geek unless you feel confident about
working at the command line.
If you can't use the Recovery Console you will most likely have to
backup your data and do a clean install. (That means erasing your hard
disk and reinstalling your system.) I recommend you show the computer to
a professional before you go this route.
Once your computer is back on its feet, and if you feel ready for a
somewhat technical challenge, you can learn about disk imaging software,
which will allow you to restore your computer even when you can't boot
into anything.