You are quite right to be wary of installing another OS. In any operation of this sort things can go wrong, and your first priority should be to be able to restore the system to it's original state if things don't work out.
The best way to do this is to take an image file of your existing setup before you begin, using Drive Image or a similar program.
If you haven't got Drive Image, you must ensure that everything is backed up, so that you could reinstall XP if the worst happened.
However, if you are careful and follow the instructions, it should be OK, and I wish you well with Linux.