I also had the same problem with debugging and OutputDebugString() starting from 2-3 weeks ago when MS updated my Visual Studio 2005 with KB971090 which is a 250MB update for 3 ATL vulnerabilities. The link below has more information and has a long list of files patched.

KB971090

I disabled my Windows' auto-update and uninstalled the KB971090 update(under the Visual Studio's Add/Remove program) which took about 30 minutes. Fortunately, I do not use MFC and ATL in my work, so I do not need this update. Although it is security update for ATL, MFC shares some components with ATL, so even the MFC and CRT(not sure why) is affected/updated. More information for the ATL vulnerabilities patched, is in the link below to see if it affects your applications.

ATL vulnerability developer deep dive

Inside the Active Template Library (ATL) Security Update Video