Whomever installed XP on a Mac should simply be hung, but that won't help you. Users are created by a user with the privileges to do so. Same for Windows as it is for Macs.
Try Startup Manager - hold an Option key as you restart the Mac. A selection of startup volumes will appear. If one of them resembles your old startup volume select it; hopefully "someone" did not delete your old OS X partition.
If an OS X partition is not available, you will need to reinstall OS X from your original system installation DVD.