Depends on the game m8, some (like il2) have all the necessary files in the game folder, and the only time it goes near the system files is for things like icons. We can get around that easily enough by placing shortcuts on the desktop etc. Some other games do hook deeper into windows and therefore need to be reinstalled after the windows install. That said, I have il2 on a separate drive for convenience, my il2 folders now run to over 100gb of gsme installs, downloads, skin packs, missons etc etc and I lack both the time and the inclination to go through installing that each time I reformat. My data (music & pics etc) is on a separate drive for security (I also back that up to two external drives - data loss can be painful).
In short, if you want to keep it, back it up.
Jabo