Yeah, that's how it worked before. But after I did the clean install, the choice was gone. Loaded latest touchpad driver from HP, still no box. Loaded synoptics driver, that brought the choice back, but it didn't work.
Disabled touchpad entirely, still worked whether mouse attached or no. Deleted synoptics driver from hard drive, still can't turn off touchpad. Etc,etc. At one point, it would work as it should (disabled when mouse attached) only until the first hibernate/wake cycle!
Managed to delete the PS/2 port driver, without realizing that would also disable the keyboard. Oops! Tried to copy PS/2 driver from the old hard drive, but that didn't work either. SFC was apparently broken with the July update, it reported problems but refused to do anything about them. Found a series of commands that was supposed to fix SFC, didn't work.
Finally had to re-re-load Windoze, which turned the broken touchscreen back on (which keeps either mouse or touchpad from working). "Lately it occurs to me, what a long strange trip its been."
That put me back to the touchpad that wouldn't die, but it finally occurred to me to "update" the bad driver (which thought the touchpad was a PS/2 mouse) with the old touchpad driver from HP (which Windows said was not compatible).
That's probably obvious to everyone else, but ...