I've never used the feature, but SuSE and Mandrake can apparently resize an NT partition to make room for themselves. Partition Magic can also shrink a Windows partition without losing the data. For safety, you should still back up your data files.
If you want to back up a complete Windows installation then the first thing that you need to decide is where you are going to store the copy. You can then use tools like Mondo Rescue, PartImage or Ghost to make an 'image' of the drive.