CD or floppy boot/rescue/backup/maintenance system. It has support for a lot of filesystem types (Reiserfs, Reiser4, ext2/3, iso9660, UDF, XFS, JFS, UFS, HPFS, HFS, MINIX, MS DOS, NTFS, and VFAT) and contains a bunch of utilities for system recovery.
If your windows installation has issues, or for some other unmentionable reason is unable to get updates from the windows server, this is what you need to do.