How do you know it is because you don't have the 100mb partition?
More to the point, why are you using Windows backup - there are plenty of others.
If you want to find out - sure you can create a 100mb at the start of the drive and make it "system". Not rocket science. It does mean you will have to move the entire contents of the 7 partition 100mb across to the right - partition program will do it for you. Seems a bit drastic - at least there is only about 20gb on there.