Press start and type "run" - in the window that appears, type "cmd" to open command prompt. Type the following in the box that pops up, one line at a time:
net user administrator /active:yes
net user administrator *
It will then prompt you to enter a password, just hit enter twice to leave it without a password.
Reboot the machine and log into the administrator account that should now be visible at the log in screen. Go to control panel and modify the settings for your personal account.
Next, open command prompt again and type:
net user administrator /active:no
Reboot the machine and log in as usual.