I have to OS both XP, I want to delete the one on C:\. Is formating the only way to do this.

Go Start -> Run. Type osuninst.exe and hit "Enter". If that doesn't work go here: http://support.microsoft.com/default.aspx?scid=kb;en-us;Q312569

That seems to really work, but what about deleting an os choice menu, I have one that say
1 Windows XP
2 Windows

So I would like to delete One of them so that I sould not have to make a choise at all because only one works....

I usually take a 1 Tesla magnet, sure to get rid of anything on that drive :)

More seriously, just boot from another medium and remove the Windows or WinNT directory and your OS is effectively hosed.
Disk might still try to boot but will do you little good.

More generally, repartitioning the disk will hose anything on it just as does formating.

So I would like to delete One of them so that I sould not have to make a choise at all because only one works....

You need to edit your boot.ini file.

http://www.tweakxp.com/article37466.aspx

The above link shows you how.

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