Grub error 17 is
Cannot mount selected partition
This error is returned if the partition requested exists, but the filesystem type cannot be recognized by GRUB.
There is no difference if you use one or several disks to put your operating systems on as far as I know - have not tested though
From the live cd you are able to mount the partitions (if you use standard Cassandra just open gparted and every partition is mounted and opened - can be annoying at times)
and make changes to /boot /grub/menu.lst
What is recommended is to use rootnoverify instead of root for Windows like
title Microsoft Windows XP Professional
For Linux check that you use the right partition like
title Linux Mint, kernel 2.6.20-16-generic
kernel /boot/vmlinuz-2.6.20-16-generic root=UUID=c7cca7f7-3cb8-48f4-8065-2eb36e4b1ccf ro quiet splash
Note that grub uses one unit less in its designation, so (hd0,1) above means that grub is in /dev/sda1 and the Linux kernel in /dev/sda2
Instead of the notoriously unreliable UUID in the example above I could have used /dev/sda2.
So check your partition numbering and adjust the menu.lst to it
The default location for installing grub is hd0 and you choose at the summary page of the install. There's a button Advanced.....
Judging by your description I think the numbering of your partitions and /boot /grub/menu.lst does not add up.