all data, including old WIndows installation, will still be on hard drive....which, if connected to an unused SATA port and powered on, should show as your D: or E: drive, depending on whether you have a DVD/CD ROM, etc...
It's easiest/'cleanest' to perhaps go get what files you want off of it (you can navigate to the desktop and various doc folders), copy them over, and then simply do a delete partition/reassign partition/quick format to the old drive, start fresh with it as a storage drive, or whatever you wish to put on it....