Adding Hard Drive

I have a hard drive with very important info. on it that I need to copy onto my new hard drive that I installed. The hard drive with the info. on it isn’t installed. It lays in an anti-static bag on a shelf. How can I retrieve the data on it? Would I need to install it as a second hard drive? And if installing it as a second hard drive, wouldn’t that erase the data on it? Any suggestions would be helpful. I have a Compaq Presario 7596 desktop model. I went to their site but they didn’t have the info. I needed. Also, I have an empty drive bay on my computer but I don’t think I can install a second hard drive on it.