Read somewhere that EasyBCD can manage the operating systems you have installed. Haven't tried it myself, but you might want to do a bit of research on that.
Easiest way is to just format the one you dont need. Then boot into recovery console on your xp CD and do a FIXBOOT and FIXMBR command, if that doesnt work then use BOOTCFG /REBUILD command. Make sure you backup any important data, as things can go wrong
one of the problems that i have with it is that both operating systems are on the same hard drive and i have this problem from a partition combiner that a friend was helpin me with.
So are you saying that they are on the same partition but in different folders? If so, then the same applies, just delete the folder with the unwanted OS. And then follow instructions in post #4
Thats not easier if he already has loads of stuff installed, it means he will have to reinstall all his programs (normally takes longer than the OS itself)