Quote: "I've only seen this problem in Windows7"
This was Vista.
Quote: "I've had the problem before, just go into task manager -> processes, see if your .exe is still running, then just end it."
I'd already tried that - no sign of anything DBPro related open anywhere.
Anyway, the problem has now gone away. I solved it as follows:
1. reboot
2. delete files I didn't need, i.e. exe, dbpro, etc.
3. create a new .dbpro project file with same name, code, etc, in same folder as before.
Then, Yippee!, everything is back to normal. Looks like either the .dbpro file or one of the Windows system references to it got corrupted somehow.
Thanks for the suggestions anyway.