I/O Error running P-FFx

I'm having an unusual problem that affects P-FFx intermittently on my work machine.

Here is the situation. I can run P-FFx from my thumb drive on my work (XP) machine, surf, etc. Everything's fine. But at some point, if I exit and try to re-run it, it crashes on startup with the following error:

Title: XPCOM:EventReceiver:firefox.exe
Message: The instruction at 0x007FDC8F (not sure if this changes) referenced memory at 0x007FDC8F. The required data was not placed into memory because of an I/O error status of 0xc0000185.

Now once I get this, P-FFx will always crash on startup, even when run it in safe mode. Removing and re-inserting the thumb drive also does not help. There are no copies of Firefox or anything like Acrobat Reader hanging around, according to Process Explorer. It just won't run again.

That is, it won't run again UNTIL I take the thumb drive over to my (W2K) laptop and run P-FFx there. It comes up on the laptop every time with no problem. I can then take the thumb drive back to the desktop, run P-FFx, and up it comes!

Other background that may be relevant: The laptop has FFx installed on the HD. The desktop HAD FFx installed on a removable HD which went south and is no longer connected to the system (which is why I put P-FFx on my thumb drive in the 1st place )

Now what is going on here? ANY ideas? I find this rather baffling. Is P-FFx trying to read something from the missing install of regular FFx and failing to do so, causing the crash? The little poking around I did with SysInternals Filemon and Regmon didn't turn up anything conclusive.