Hi,
We have what would appear to be the same problem on our systems.
It was caused by the installation of a Windows Update:
Microsoft .NET Framework 3.5 SP1 Update for Windows 7 and
Windows Server 2008 R2 for x64-based Systems (KB982526)
After it was installed TestComplete failed to recognise .NET properties in the object browser or call extensions through 'dotNet'.
We rolled back our systems using System Restore to before the Windows update and all was well again. I assume we are having problems because we're still on TestComplete v7.51, hopefully this is resolved in an update to 7.52?