As I could understand your requirement, you wish to password protect a single partition in your Windows 7 system. Moreover, you would want to do this without installing any third party programs on your system. I've understand that much pretty well.
The Good news is that you don't really have to install anything extra on your Windows 7 machine as it by default comes with BitLocker protection, which enables you to password protect one or more drives when needed. However, not all Windows 7 versions come with encryption feature. Please confirm if your Windows 7 Operating System is actually Windows 7 Pro or Windows 7 Ultimate edition, or not. If your system is running a Home edition of Windows 7, I am sorry to inform you that it does not have BitLocker encryption feature. (And it may not be even downloaded from somewhere with any hack)
An easiest method to look for the Windows 7 edition you are running, is to use this keyboard combination (Press and Hold Windows key and press PauseBreak key) on your keyboard. The window that appears afterwards contains the details of the Windows edition you run.
However, if you have a Pro or Ultimate edition listed there. You can open Windows Explorer (Windows key + E), right-click on the partition you wish to encrypt and further select 'Turn on Bitlocker'. That's it. The rest of the process is as simple as feeding in your desired password and saving the 'recovery key' that it provides at the end of this process to a safe location (on a pen drive or maybe just on you desktop). Whenever you forget your password, you will able to recover your locked data using this recovery key, hence to kept safe.
Also, another time saving point is that whenever you turn on Bitlocker encryption on a hard drive partition, it may take a considerably huge amount of time based on how much data has been stored on that drive. Therefore, enabling Bitlocker right when your drive partition is fresh and empty will be the best deal. And then it may take just a few seconds to complete.
Note- If you Windows 7 edition does not support Bitlocker encryption, you can consider an upgrade to Windows 7 Pro or Ultimate. Also, if you'll be considering a higher version, say Windows 8 or 8.1, please double check if the edition you are getting supports Bitlocker or not. For example, I am running Windows 8.1 Enterprise and it does come with Bitlocker. Alternatively, you can search online for the best Bitlocker alternatives. From my experience, I would suggest you grab Folder Lock, as it is reliable and enables you to lock not just folders but files as well as drives. They would also provide you with all the possible help in case you forget your passwords with your data locked inside. Obviously, it is a paid software yet very reliable. You may get a trial period to use and understand the software before your decide to whether buy it or not. Please find its official download page below.
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