To remove an o/s you do simply format the hdd. However, if you've setup a dual boot you will need to remove the entry for the removed o/s from the boot.ini.
The complexity of this depends on which o/s you are keeping.
To edit XP boot.ini simply run system properties -> advanced -> startup and recovery settings ... and edit the boot.ini to remove the entry for the other o/s.
To edit the Vista boot.ini you need to use the command line tool bcdedit.exe.. which isn't easy.. alternatively there's an application out there called Vista Boot Pro which makes editing the boot.ini very simply in Vista..
you could also boot off the Vista DVD and perform a startup repair, which may sort it out.