Hi,

My friend had to clean-reinstalled XP pro on his laptop because the cpu was running at very high percentage.

Now after reinstall, the cpu is running at 50% constantly.
Does anyone know what is wrong?

thanks in advance

Open Task Manager/Processes
Click the CPU column to see what processes are using the most resources. Same with the memory column.
Have you installed all the Windows updates?

Could be a Virus/spyware causing this. If you find out the name of the process draining all of your resources, enter it's name into this handy website.
Has helped me understand what has been causing any of my sudden CPU 'draining'.

edit: If the link fails to work go here: http://www.processlibrary.com/

could be an incorrect driver installed for your chipset make sure your usng the manufactures drivers for all hardware not microsofts drivers! It is very unlikely to be a virus if its a clean install, on the other hand it could be a hardware fault!