You can install Scientific Linux on the secondary hard drive. In order to be able to boot it on the second drive, you will need one of the following (and only one):
1) make a boot floppy or boot cd to boot Scientific Linux, and install grub to the root partition (not the MBR).
2) install grub in the MBR, and use grub to select either Scientific Linux or Vista to boot.
3) configure Vista's ntldr to boot either Vista or Scientific Linux.