OS X Snow Leopard won't boot into Recovery mode, need to reset admin password
Mac OS X Lion, 10.7 was the first OS to introduce the Recovery Partition.
Internet Recovery came later still, in Macs introduced in 2011 [iirc].

That means your only way to boot from other than the regular boot drive is to either use the original Snow Leopard disk, or attempt to boot it from a USB key.
That in itself may depend on precisely which Mac Pro it is. Personally, I have never managed to boot a 2008 3,1 from USB, though others say they have succeeded. I've successfully booted 4,1 &amp; 5,1 Macs from USB. I've never owned a 1,1 or 2,1 to test.

As it is a Mac Pro, you have the potential of a third option - you can drop in a new drive with either a full system on it, or a bootable installer.

If all you need to do is reset the password, then that can be done on older OSes from Single User Mode...

From MacYourself - How to reset your Mac OS X password without an installer disc


  
  Power on or restart your Mac.
  At the chime (or grey screen if your chime is turned off), hold down Command+S on your keyboard to enter single-user mode.
  This step is optional, but it’s a good idea because it checks the consistency of the hard disk before moving on. At the prompt, type
  fsck -fy and press Enter/Return. Wait for the checks to complete
  before going to the next step.
  Type mount -uw / and press Enter.
  Type launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist and
  press Enter.
  Type ls /Users and press Enter. This lists all of the usernames on the computer – helpful if you don’t know or remember what these are.
  Type dscl . -passwd /Users/username password and replace “username” with one of the users displayed in the previous step. Replace
  “password” with a new password of your choice. Press Enter.
  Type reboot and press Enter.