Unexpected computer oddnes

[Josquius]

I've came home and tried to turn on my computer, the hard disks are detected as normal and all seems fine but...I'm took to a win xp login screen rather than windows from the start. This seems very strange to me for a number of reasons.
1: I'm pretty sure I didn't setup the computer to need this before I went
2: I wouldn't have called it default 'Administrator' if I had
3: None of my usual passwords work.
Now it could be that I just forgot all about settin the computer this way before I left just in case of snoops, I could have been too lazy to change the admin name (or wanted to confuse snoops) and decided to try a new password when I did it but...this is all unlikely.
The only thing I can think of happening to this computer since I left is that I allowed my sister to borrow my graphics card for her computer whilst I was gone, its been working fine there and its obviously working somewhat now its home as I have picture. But still. Its the only thing. Could this mess things up?

Any clues on what to do?

I was thinking of installing windows 7 anyway this christmas just to play around but I somewhat suspect I would need to reformat or know 'my' admin password to do this. Pah.

[DontSayBanana]

You would need admin privilege to do a system upgrade.  It's possible to be set up this way, but not typical.  I'm a worrywart, but I'd say restart the computer to make sure you're logging into the "Administrator" account and not the "administrator" account- I've seen at least one virus that created an additional "administrator" account while the real Administrator was hidden and used that to farm data from what the user thought was a secure account.

[Josquius]

I found a programme to hack the password.
I've still no clue what caused it to be there but....all seems fine now....