|If the computer gets to the point where you see the initial Windows graphics, but then the computer restarts....|
Win XP is set by default to automatically Restart when it encounters an unrecoverable error.
If you can't get into Windows ....
Remove any bootable CDs or DVDs you have in drives.
Boot the computer and starting very early, press F8 repeatedly, do not hold down the key, and when the "Advanced..." Windows loading choices menu appears, choose
Disable automatic restart on system failure.
That will attempt to load Windows normally.
It's a one time thing - it only disables Restarting on system failure when you have selected it.
Some computer's bioses have their own use for pressing F8 while booting. E.g. you may see a screen where you can select which device to boot the computer from. If you see something like that, READ the screen and find out which key you need to press to exit that screen, e.g. Esc. Press that key, then immediately repeatedly press F8 until you see the 'Advanced..." Windows loading choices menu.
If a blue screen appears it will stay on the screen.
Copy down the things that are unique in the message and post them here.
STOP: 0Xx00000xx (we don't need the stuff in brackets beside that)
There may be a problem file named at or near the end of the text.
Quote that if you see that.
There may be a link to More info or similar - if so, click on it to see if it names a problem file.
There may be a mention of a MINIDUMP file having been made - if so, that file can be analyzed.
NOTE that a common reason for a blue screen error is the ram has a poor connection in it's slot(s).
Remove memory / ram sticks & clean the contacts.