Excel was started in house, yes. It actually started on the Mac rather than the PC. For a long time the Excel team didn't even use Microsoft's own C++ compiler to build their software. They had their own compiler to target both Windows and the Mac.

I know Access was bought in from outside. I'm fairly sure that Word (as in WinWord) was in house though, not that the first version did much.

Even VB was not started in house. Neither were Visio, SQL Server, Flightsim, SourceSafe, DOS etc, etc

They certainly are good at buying up other people's code...