It's at least a good chance Knoppix won't be your hero unless you make Google and forum search your good and trusted friends..
You should look it up, the very short versions are:
If only booting is affected, you should be able to boot Knoppix from CD/DVD/USB, mount the actual Windows volume(s) and retreieve data from them.
If the file system is damaged, most will advise specialized recovery/repair tools, those tend to run under Windows.
If the disk is also severely damaged (more than a number of bad blocks), data may still be recovered, but that's an expert job.
And with the current price of USB sticks and external USB harddisks, I really don't understand why you don't have backup.