I had Bitcoin qt installed on my C drive and I downloaded the blockchain onto an external hard drive. I recently had to re-image my C drive. What I want to do is reinstall the Bitcoin software on my C drive without having to re-download the entire blockchain again.
Since you downloaded the blockchain onto an external drive, you do not need to redownload the entire blockchain. When you start Bitcoin Core for the first time, it will ask you to choose the data directory. Just choose the folder on your external drive which contains the blockchain data. That folder should have a folder named "blocks", a folder named "chainstate", a file named "wallet.dat", a file named "debug.log" and a few other files.