Firstly, are you working from a Manufacturers restore disk or from a genuine winxp cd? Do you get to see the Windows boot logo and then it goes off? If that is the case, then try loading the OS in safe mode (after bios screen keep hitting F8 to get load options) and set the resolution down to 640x480 or something low, then restart the machine and see if that works.

Good advice from veggii if the above is not the issue, checking all connections inside if you have tinkered with anything in there, if not, it still won't harm making sure all of your cards and cables are seated correctly. Also, make sure you will not be voiding any warranties you have with your machine before delving inside.

Do you get to see your BIOS screen when the machine starts up? If not, check your graphics card is seated firmly in place and check your monitor cable connection. If everything is seated nice and snug, see if you can get a hold of another card and try that. If you have onboard, give that a whirl as well. if you still can't see anything, possible mobo issue.

If you can see the bios screen, does your machine then hang after this? If so then it would seem that your machines partitions could have a problem. Go in to your bios and set your first boot device to CD-ROM. You can leave this as the first boot device as if there is not a bootable disk your drive, your machine will try the next device in your list, generally hard disk. Even if there is a bootable disk in your drive such as a windows cd, just let it skip past the screen saying press any key to boot from cd.

If you are using a manufacturers boot disk, you should just be able to let it work away setting everything back to factory settings. If you are using a genuine bootable windows disk, run through the installation procedure again and as ThaRaven mentions, delete the windows XP partitions and follow the on screen prompts to create a new partition and install windows on the new partition.

Hope this helps a bit and let us know as much info if you need any more info.

Cheers.