someone has installed windows XP on my I-Mac and I can't log on as any user

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.

Hi,

What are you trying to do, startup in Mac OS X or Windows?

When restarting, hold down the option key until you see a your hard drives and let go. You can click the drive to either boot into Mac OS X or Windows.

Let me know how it goes.

Hope this helps.