The .pdb cannot be written if it is in use by another program.
I can see 3 possibilities as to how this can happen :
- Two instances of Visual Studio 2008 are working on the same project (unlikely, I know)
- Some source control plug-in in Visual Studio 2008 has decided to also manage the .pdb, so one needs to delete the reference to it in that plug-in.
- Some files included in the solution are missing, so one needs to remove their references (ie. the files with the warning sign).