Windows 7: issue observed in Save Dialog Box in Windows 7

Hi,
In our application, We are providing the default file name without extn while opening save dialog box. Once Save button is clicked, We are appending the corresponding extn and save it in the folder. This is working fine in Windows XP OS.
But, in User Login while running the application in windows 7, the Extn is automatically appended while opening the save as dialog box. When it is saved, it displays the error "Invalid File Name".
In Admin Login, default filename is without extn in the save as dialog. But it also displays the same error "Invalid File Name".

Which causes for this unexpected bahavior in windows 7. Please help me in this regard.

Regards,
Narayanan

what is the filename your using?

Well it not being Windows XP should be a big clue.

File Name used in Save as Dialog Box: Con_LTZ_20100225_211231
File Filter used (Only One option): SEM Files (*.sem)

.sem is extension used for storing our files.

One more observation here is: In admin Mode, .rem option is not appended by default. But the issue is, still it says "Invalid file name" while saving.

Any registry options or any settings change, will help to resolve this issue?.