Could be a driver problem if it loads up in safe mode. After your BIOS screen comes up,and befor ethe XP logo comes up, press F8 until a boot menu comes up. Should be an option that says "Disable automatic restart on system failure." Select that, and let it go. You can also do this from within Safe Mode by doing this: Start > right click My Computer > Properties > Advanced tab > under Startup and Recovery click 'Settings' > Uncheck the box that says "Automatically restart" > click OK on all open dialog boxes. Let us know if you get a BSOD (Blue Screen of Death), and post the following info:
The words in all caps at the top (for example: IRQL_NOT_LESS_OR_EQUAL)
the STOP error code at the bottom (formatted assuch: 0x000000## where ## = some alphanumeric combination)
Any reference to a driver file (usually a file name that ends in .sys)
Also visit this link to figure out what it is yourself if you wish:
http://www.techist.com/forums/f9/how-troubleshoot-specific-stop-errors-193301/