No matter if you've got Windows XP, Vista or 7; if you forget the administrator password for the system, there are very few ways to get back in again. In fact, the only two ways to regain access to your PC if you do not have the password any more, is to either reinstall the system, or to "hack" it with a special piece of software.
First, let's talk about reinstalling your system. Windows is just a big program that runs on your PC. This means that you can "reinstall" it to gain access to your computer again. However, this presents a HUGE problem – reinstalling Windows will literally wipe your hard drive and will leave your computer without any of your settings, files or programs that you had installed before. This method is not recommended if you want to keep your files / programs … and is only used by a select few people who just want to get access to their PC again.
The more robust method to regain access to your PC is to use a special piece of software, known as a 'password resetter' tool. This software basically loads up before Windows loads and then "hacks" the files inside your system that contain the password. We're lucky in that all versions of Windows keep their passwords stored in the same way, meaning that you can use one of these programs to load up onto your Windows 7 system, change the various files & folders that store the password, and allow you to log in again.
This technique is preferred by most, and is the one that will reset the password of your PC without any further problems. Of course, you could also reinstall your computer – but that would lead your system to have none of your files / settings inside. You can use the password reset software to quickly gain access to your PC again, by letting it "hack" your system's files & settings.