I bought a new 3TB Seagate Desktop internal hard drive and wanted to add it to my hard drive that's already installed but my computer won't recognize it. Please help!

How old is your computer? Is it the BIOS that doesn't recognized it, or the operating system (which one)

As suggested by Ancient Dragon, the size may be a problem. Most older BIOS chips only handle up to 2TB devices. Have you looked for a suitable update from the system manufacturer for the BIOS?

I'm not for sure how to tell how old my BIOS is? My operating system windows vista. I don't hear anything running when I hook it up. I have an extra slot for an addition hard drive and plenty sata slots for extra drives.

If you have 32-bit Vista then the max size of 2TB -- you'd have to partition it into smaller partitions to make it useful for 32-bit operating systems.

To check the bios, during reboot and before Vista starts press the Del key (or whatever key the computer tells you to press) to get into the BIOS. Once there you will have to browse around it to find the manufacturer's ID or part number, then look it up on the web. The BIOS might even tell you the date it was made. Contact the manufacturer to see if there is a problem with that 3TB drive.

If you are in USA you could take it to Best Buy Geek Squad, but it will cost you some $$$.

I did find out vista don't support 3TB hard drive and was told I had to get a different operating system. But I did find out it was a 64-bit. What os do you suggest? I was told windows 7 was good.

Either Windows 7 or 8.1 is ok -- 8.1 is radically different than any previous version of Windows, but it has other desirable features, such as quicker bootup time. Before buying either you need to download and run Microsoft Compatibility check program. Click here and here And here is a list of some of the differences