safe mode

[frequency]

hey guys whats up.....my friend just bought a used computer that had windows xp home edition. he didnt want home edition, so he went to the store and bought himself the professsional edition. during the 3rd step, which is belive is installing the devices....it stopped at the 50% mark. so we took the disk out and cleaned hoping that would solve our problems but nope. it didnt work...everytime i restart the computer it immediately starts off where it left which is installing the devices after the boot sequence. it starts reading the reading the cd rom data and after it hangs at 50% for like a very very very long time and plus the disc stops spinning like it cant read the file or something or some crap. so i was wondering how do i go into safe mode cause i want to go back before this even happened and do a system restore check and forgot this ever happened....lol any help is appreciated