I've got a rather mixed situation going on. I've purchased a copy of Windows XP Home and I have the lamenated pamphlet with serial number to prove it. However, I didn't take very good care of the CD and after awhile it started to crack and warp to the point where my PC couldn't read the thing properly so I through it out. Of course, Windows is too fucking expensive for me to buy a brand new copy, and I was bound to need to do a reformat eventually. So a few days ago I downloaded a copy of XP Pro, reformatted and installed it onto my C partition, and then realized I had a Backup file from awhile ago. I'm guessing my Windows is not the most stable thing right now (two security updates fail to install and it's having trouble identifying some hardware) but since it isn't mission critical at this point I hardly see the point in worrying about it.
Jam it back in, in the dark.