I just added a time capsule to the Mac. Now I would like to take the My Book and use it to back up the Vista machine. How do I change it from Mac to Pc?
Reformat the My Book on your Mac to FAT32, here is a guide: http://sites.duke.edu/oit-mps/2010/11/10/pc-to-mac-external-hard-drive-formatting/
If you only want to use it on the PC you can format it on the PC to NTFS after it is readable in FAT32. Then the PC can read and write and the Mac can read only.
And i would not trust my only back up to a My Book or any other single drive if the information is important.