To downgrade from 10.3.6 to 10.3.5, recover from your clone backup. In the future always make sure to clone your operating system prior to making upgrades, if you didn't this time.
The other way to downgrade to 10.3.5 is to shut down your machine, detach all third party devices, Archive and Install from the system CD that came with your machine, or the retail 10.3 installer CDs (the black ones with the white and gray X logo that do NOT say Upgrade or a Mac model name) (whichever CDs are newer*), repair permissions, and then run the 10.3.5 combined update:
Repair permissions is in Applications -> Utilities -> Disk Utility -> select hard disk in Disk Utility -> Click on First Aid.
Repair permissions when done.
* Remember you can't install the retail Panther (10.3) CDs, if your machine came out after Panther, and must use the CDs that came with your machine.
I love OS X, but it's pretty darned sad that experts recommend a full system clone prior to a point upgrade. Of course if one cloned to a firewire drive, and the upgrade disables the firewire drive, a clone alone might not help. One needs both a clone and a boot CD.