Spoilers advice is good. However, you want to make sure that you have not already destroyed or corrupted data within your operating system or programs. Otherwise you will just be backing up all of the corruption and that corruption could lead to further data breakdown on your new drive.
Personally, I only back up my actual user data. That is all my word docs, spreadsheets etc. my pictures, music, and anything else that can't be replaced from a program disk. I reinstall my operating system and all of my programs from scratch.
Another reason I go the long route and reinstall everything is because that over time, your hard drive gets fragmented, your registry grows to insane sizes with stuff you installed, deinstalled but it never quiet gets cleaned out fully. You will just be transferring all of these things that make your whole system run slower. Even if you defrag regularly, it never gets back to as good as it gets with a clean reinstall.
Backing up a particularly hard to install program or one that takes forever to customize back to where you like it, is an option. But if it writes stuff to the registry, then you are forced to back up the OS along with it or you will have to reinstall it anyways. Most programs write to the registry.
I have one game that is HUGE, hard to customize, and actually does not rely on writing to the registry. This is the only item for which I maintain a backup of this on a separate drive in case the first crashes.
I know it's more work, but I bet if you reinstall everything, you will find your PC works a lot faster. If you hardly ever use your PC, this might not be an issue but if you are like me and you are constantly adding & deleting programs and data, wiping it clean once every year actually is almost required, hardware failure or not, to keep it running at it's top speed.
