This happened to me before.
The only difference was i was in 10.4(Tiger), not10.5(Leopard.) What you need to do is:
1.Go to Macintosh HD/System/Library/CoreSevices and delete your dock file (Note: May damage Hard Drive)
2.Insert Mac OSX Leopard installation disc, backup, and reboot the Mac.
3.(During startup)Hold the "C" key, and wait for the Aurora backdrop and the language selection to let go
4.As far as I know, re-install, but DO NOT ARCHIVE!! If you DO archive, you will have to go through the install AGAIN.
5. Drag your backed up files where ever. Your dock will now be restored.
The problem is:
If you have been going to Leoparddocks.com, and downloading those docks, you may have, on accident, deleted the entire contents of the dock's rescources folder (right/ctrl-click, then show package contents, then rescources) You are only supposed to replace the dock images.