Free software I use:
The backup will be out of date within the month. You should backup your system as a whole but don't waste time backing up installation files. Get the latest one next time you need it.
Commercial software I use:
I'm confident that I'll be able to download it again and apply the licence I've paid for. I still have a few CDs for old games that I might want to revisit.
Software I write:
I do enough development to justify a separate file system for my source code. I have a small selection of folders for binaries that I add to my system path.