Overheating or a weak power supply can do such as this. Open the case and clean out all the fans with compressed air. Turn off the PC and disconnect the fans from the system first. Check the ratings on the power supply. Depending on the PCs hardware configuration certain wattages are recommended as minimums. An older supply can be too optimistic as to it's output. Also, posting more information about the system would be a helpful addition in determining the possible cause of problems.
The problem is on my daughters computer running Windows XP.
This started a few days ago. The screen will go off then after 10-20 seconds will come back on again (during this time the moniter switches into standby mode) when the screen comes back on the system is frozen. It will not respond to anything (tried esc and ctrl, alt, del) It will freeze after a matter of seconds after reboot. I have tried safe mode and it seemed to be OK but to be honest I haven't a clue what to look for so all help will be gratefully appreciated