If your computer came with Leopard, then you will NOT be able to install Tiger on it. If it came with Tiger (or earlier), then you're in luck. Unfortunately, the Mac OS doesn't support "live" partitioning except through Boot Camp, so what you'll have to do is this:
- Create a full backup of your system, either via Time Machine or with a disk cloning program such as Carbon Copy Cloner (I'd recommend SuperDuper! but it's not Leopard-compatible yet).
- Boot your system from the clone (if you made one) or the Leopard install DVD (if you didn't).
- Open up Disk Utility and partition your drive. Note: This requires erasing the drive - that's why you made a backup.
- If you cloned your system, clone it back to the Leopard partition. If you used Time Machine, reinstall Mac OS X then, when prompted, choose to restore your system from the Time Machine backup.
- Install Tiger on the other partition.