Thanks for the replies. I got it wrong, it's Vista I am trying to open, so the CTRL-ALT-DELETE trick didn't work. I tried the SystemrescueCD but it opens as a text file so I am too dumb to use it. Then bought the Windows Geeks one, which looks good but I do not have a Win32 system here to open it. (I use Mac.)
I'll try and borrow a Win32 machine to burn the image file to disc then transfer to the locked computer. I am way out of my depth here and may just buy a new machine.