I believe I know what causes this: POIDE has a problem with read-only source files. Check to see if the source file causing the problem has the read-only attribute, and remove the attribute. Then go to the directory
C:\Documents and Settings\<your user name>\Local Settings\Temp
and delete the Pelles C backup file (it will have your source file name and extension folowed by another extension with a letter and a random string of digits). The problem should go away "forever" - until you have another read-only source file...
The following sequence illustrates the problem with POIDE:
1) Open a new project, call it A
2) Create a source file in that project and save it
3) Switch to some other project (project B)
4) Go into Windows Explorer and set the source file to read-only
5) Switch back to project A. The source file will come up in the source window as expected.
6) Switch back to project B. POIDE apparently tries to do something to the file at this point and hits the
fact that it is read-only, which causes POIDE to not delete the backup copy.
7) Switch back to project A - voila, you get the pop-up screen and the refusal to load the file into the source
window.