It will be difficult to get the error message because the system is restarting before he can write it down. When Microsoft released Windows XP in 2001 they included an automatic restart feature that will reboot the system in the event of a blue screen of death. Unfortunately, if you get a blue screen every time you boot, then you get stuck in a loop.
First, I would recommend that you run a Windows XP repair installation. Boot from your original Windows XP CD. When the first prompt comes up, press enter and then press F8 to agree to the terms. On the next screen select R for repair, and sit back and enjoy the ride.
After the repair is compete, the computer should boot normally. Before you do anything else, you will want to disable the auto restart feature.
To do this, click on Start and then right click on My Computer. Next, left click on properties and then left click on the Advanced Tab. Now left click on the "Settings" button under Startup and recovery. Uncheck the "automatically restart" option.
You may have to reinstall some drivers and windows updates once this process is complete, so have the disks for modem, video card, motherboard and network card handy!