I suppose you tried without administrator password: just press ok when it asks that password; if you are a bit lucky, no one has set up an administrator password.
If you don't want to save anything, just insert the windows xp cd and reinstall Windows XP. You can erase the partition, recreate and install on a fresh partition. Be careful, still: you need all the drivers for all those components you see, outside computer case, and also for the components you don't see, inside computer case. And, of course, you will have to reinstall every application you want to use.
