Hi - Thanks for your replies. Is it possible to get to the Command Prompt if I cannot log onto either account ? I've tried Safe Mode, but even that asks for user passwords, which I don't have. There doesn't seem to be an option in the Startup menu to go to Command Prompt only.
I will check out the password recovery system from the website mentioned by turk_1000. I was keen to try Ophcrack - but there doesn't seem to have a LiveCD option for Windows 7 - only for XP and Vista. The other option seems to be a program which needs a logon to run.
If you can offer any further assistance, please do reply.