I got this fix from this forum so who knows if it's OK or not?
Boot up in Safe Mode (usually by tapping th F8 key as your computer is loading up).
At the log-in screen you should then [hopefully] see a user account called ADMINISTRATOR - this is where the fingers crossed bit comes in - hopefully you never set this account with a password (just press Enter to find out) OR you can remember what password you allocated - assuming you can then log on to Windows you can then simply go back into start> Control Panel> User Accounts> find your log in name (eg wheatley) and reset the password - if it asks for the old password, then you will have to create a new account (eg wheatley2) and give it full admin rights.
Then reboot your pc and allow it to start up normally and log in under the appropriate user account.
-------------------- OK OK I can't go any faster! aaaaaaaaaaaagh!