Once you are able to determine that the hardware in your computer is one of the reasons your computer may be running slow, you can perform a basic diagnosis to find out what exactly you may need to fix, replace or manage.

To access some basic diagnosis features, click on the Start Menu, navigate up to Computer in the right side bar and right-click on it, then click on Properties in the drop-down menu.

This will bring up the Properties window, in which you'll click on Windows Experience Index under the System heading.

The Windows Experience Index will give you a basic breakdown of how your hardware has been rated by Windows. It will show you the lowest rated item on your system. As a point of reference, the Windows 7 rating system is on a scale from 0 to 7.9. In the video, the computer's Primary hard disk is rated 5.9 and is the lowest rated item in the system. To view more details, click View and print detailed performance system information. 

As you can see, it seems that this computer has enough free space, but the issue may have to do with the hard drive's speed. So, when looking for a new hard drive it would be important to consider one with a faster read/write time. Also, you would want to consider whether it's a solid state hard drive or a standard hard drive. If the hard drive is currently an IDE hard drive, you may want to upgrade to a SATA hard drive.

IDE hard drives transfer data at speeds ranging from 5MB/sec up to 133MB/sec. SATA hard drives, on the other hand, transfer data at speeds ranging from 150MB/sec to 300MB/sec.