Drive letter changes depending on how you view them

Details:
Two SATA HDD's, each with two partitions. No Raid.
First partition on each is bootable and has XP loaded. I boot to one or the other by changeing the order in the Bios.

If I open 'My Computer' the list is;*
C: Backup1, D: Main1, E: Storage1 & F: Archive1

If I type "\\Videobruce1" in the address bar of a Explorer window I get;
C: Backup1, D: Archive1, E: Main1 & F: Storage

Why is this, how can I correct this and prevent it from happening again?