Erasing 1 of 2 OS?

How would I go about doing this?
Originally, when I bought this computer back in '02 or thereabouts, it had Windows XP Home edition installed on it.
Well, I ran into some problems a few years ago (viruses, etc.), and had somebody from the local Best Buy come out to fix it (he worked there, but also had his own business on the side for fixing computers). He installed Windows XP Professional edition, and said it was better. It was, for awhile I guess.
When you boot the computer, you have the option of using the arrow keys to select which OS you want to boot (Home or Professional), in a certain # of seconds, etc.
For awhile, Professional was the only one that would start up when prompted. If you tried to go to Home, it would boot for about 2 seconds, and then just totally restart the computer. Then, one day, Professional wouldn't work, and I have to use Home everytime.
But now, it's gotten really annoying. I have like 4.19 GB free on my C:\ drive (with 33.7 GB being used). What the heck? I've tried erasing as many things as possible (things that I know could safely be erased), but it seems like nothing else will work. Its just GOT to be because of this other OS. Would anyone else agree?
Since the one I'm using now, the Home OS, was the original one installed on this PC back when I bought it, isn't there some way I can safely erase the other OS? Should I call GeekSquad (lol) or something? Surely I need to back up this one totally, w/ everything I need on it, before attempting to do anything.
Anyways...if anyone has any advice...please let me know. Thanks.

 

Firstly, you need to set it to boot from the OS you want.

Start > Right click My Computer > Propertiers > Advanced Tab > Settings under 'Startup and Recovery' > Edit > Delete the whole line that has Home in (if you want to remove the Home edition) or delete the whole of the Professional line (if you want to delete Professional edition).

That sorts out the boot setup, then just go into My Computer, and delete the Windows folder that has the O/S that you want to be rid of.