Wayne Hortensius says:
It's a known problem with no cure, just a workaround (checked the discussion forums at MicroChip's site). Deinstall MPLAB and reinstall it with everything (programmer support, emulators, the works). The seems to make the problem go away in 99% of cases. It worked for me on 3 PCs, running WFW3.11, Win95 and Win98.
Microchip says:
V5.11 has a problem with projects if PICSTART Plus and PRO MATE drivers are not installed. Go to the Microchip web page and just add those elements to your current installation and everything should work ok. We're getting v5.11.02 posted up there now which does not have this problem.