|There is usually a way around this.. |
Reboot the computer and press F8 to bring up a startup menu, select Safe Mode. The default profile that boots up is usually Administrator (with no password), click on the profile and it should let you in.
If this trick doesn't work, then you have to question the person who gave it to you or install a new operating system. There are small hacks and things that can reset the password for you, but I haven't seen one of those go smoothly without repercussion.