DELL OptiPlex GX260 bootup problems

I work at a secondary school and after recently installing XP SP2 we found that a small number of the machines had not installed it correctly and we were advised to uninstall as the system could be unstable. However, once uninstalled the system rebooted and after showing the DELL bootup screen the system rebooted again… and again, and again. The machines are now locked in a continuous cycle of rebooting, and obviously this is a major problem as the number of students now outweighs the number of machines available for use. Any advice offered would be much appreciated.