I tried the workaround, running msiexec. This did not add anything in the start menu, and Add/Remove programs did not show Graphviz as installed. I could run GVEdit but not dotty.
Checking dependencies of dot.exe, MSVCR80.dll was missing. I noticed versions of this were shipped in Graphviz-2.26/Windows, but apparently not installed successfully.
Before installing Graphviz, Visual C++ 6.0 was installed,
so older runtime libraries were in Windows/system32, i.e. MSVCR71.dll
Workaround (successful) -- download vcredist_x86.exe from Microsoft to install the VC2005 redistributable libraries.
I noticed vcredist_x86 installed the libraries in Windows/WinSxS.
Maybe you need to use WinSxS to avoid collisions between versions.