User Accounts have a mind of their own? Help!!

I have no idea of what just happened. As I was logging on to my User Account, I noticed an additional Administrator account that surely wasn't there the last time I logged on. I had no idea what caused this strange account to come up. Trying to figure out more about it, I tried entering the account, but it had a password. And no, there was no 'blank password' already set up. I went back to my account to find that I had been knocked down from having administrator privileges, and now was stuck with the standard account. And of course, right when my iPhone breaks down and needs the latest iTunes update, I can't download it because I don't have my regular Admin privileges. Re-capping, there's another account randomly showing up, taking my privileges away, and locking itself with a password. So I can't delete it! I'll have you know I'm the only one on this computer, too. My old roommate's User Account, which had no password, was now also locked up with one! And I contacted her, asking her what she might have set the password to. I discovered that she had never even set a password. On the second try of logging into that account, it was now asking me to reset the password. But it still required an old password!! The account originally had standard privileges, but now, for some reason, I couldn't delete it from my account. Oh, and the icing on this disastrous cake is I'm leaving for a three-week vacation in a few hours, with a non-functioning iPhone and barely-functioning computer. Please, any help?

Ok I'm helping you from my iPod touch at bedtime so help from me right now will be limited, but since it's so urgent I'll try to do some basic stuff before you have to leave and see if it helps. First of all, is it possible that someone was able to get physical access to it since you last used it and could have messed with it? Being locked out and having your account demoted to standard sounds like a hacking of some sort (or a seriously nasty virus) except for the fact that an account has reappeared that used to actually be on the machine. Question 2: what is the name of the new account? Is it just "administrator"? Unfortunately I don't have access to my computer right now or I could give you more in-depth help, but if there's a way you could wait approximately 8-9 hours, (I should be asleep in this time zone) I can provide better help. Is your computer a laptop you can take with you or something?

Ok I'm going to crash so I'll just post a couple ideas for you to try. If you have the original OS install disc or a repair disc put that in your computer, restart it, and then use the repair options to try and fix your computer. If you're fortunate enough to have a restore point from before the problem, you can go use it by finding System Restore in Control Panel, or just search for it in the search bar in the start menu. System restore is the easiest and most surefire way to solve it, if you have an available point to restore to. You canals try enabling the default admin account and using that to fix your problems (assuming the one that recently appeared isn't that one). Google it, sorry I can't explain it right now.