GRUB allows the booting of more than one operating system. Normally it will be installed to the 'Master Boot Record' on the first disk drive. Here it can be installed to the MBR of the drive on which the '/boot' directory of the new installation is found - which may be the root ('/') partition, or else a partition of its own (in which case it will not contain a directory called '/boot')!
Alternatively, GRUB may be installed to the partition containing the '/boot' directory of the new installation (as above, either the root partition or else, if the '/boot' directory has its own partition, the partition mounted at '/boot').
A further alternative is to boot the new Linux system from an existing GRUB set-up by adding the entries for the new system to its menu.lst file. At the moment this must be done manually, as it is not easy to know how the other system handles its configuration files (they might be generated automatically, overwriting changes).
Existing bootloaders may also be linked from the new one, if this is installed to the MBR. If these are also GRUB (having menu.lst) an entry for their configuration file can be added to the new menu, otherwise they can be chain-loaded (e.g. GRUB2 or NTFS).
If you have mixed IDE(PATA)/SCSI/SATA devices attached it is quite possible that GRUB will get confused about their order. In other words you might find that the association of GRUB names (hd0, hd1, etc.) to Linux device names (/dev/sda, /dev/sdb, etc.) is incorrect. The result is probably that you will not be able to boot your new system. In this case you will have to manually edit menu.lst and put the correct number after the 'hd' device entries.