Anyone seen this before and/or have a solution (other than make sure you edit function library files in textpad or similar)

I have a function library file built up over some time.
This file works fine - the tests built with these functions run
I now need to run tests on a new machine - so I install QTP, copy over the required files. Run the tests previously developed - the tests run fine.
I need to make one small change - editing out a MsgBox line previously entered for debugging purposes. I open function library in QTP and comment out the offending line and save
Rerun tests - tests now wont run at all - get a
>Error in loading VTF application Library
>Error Description: Type mismatch
>Error Code: 13
>Error Source: Microsoft VBScript runtime error
>Exit test

I check syntax - all looks fine, but finally notice that the function library file has doubled in size - looks like save in QTP has corrupted the file.

I recopied the original file onto the machine - made the edit in textpad - and ran tests - all works fine.

I have tried reinstalling QTP - I get the same corruption problem.

Is there some setting I am missing - I really dont want to have to drop out to textpad or similar each time I need to update functions when I'm developing tests