To shrink existing partitions to make room for a new one, use a
third-party partitioning program like the free Partition Logic.
After you install Windows Vista, the operating system’s boot menu
will offer you the option of booting Vista, or your previous
Windows version.
Note: This software comes in two versions, one for booting from a floppy and one for booting from a CD-ROM image. This is the CD version.
–Scott Spanbauer