The most likely cause for this is that Maple is installed on this computer.
The main culprit we know about right now is the Windows registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\AppPaths\excel.exe
It contains a value called "Path" which the SigmaPlot installer incorrectly assumes will only contain the Excel path. We know that Maple puts its own path there and that can cause a problem, compounded by the fact that Maple doesn’t uninstall this value.
A temporary workaround is to edit this key so it only contains the Excel path and restore it after doing the SigmaPlot install. If you are comfortable working with regedit (Windows: Run regedit. Click the Windows key bottom-left, write: regedit, and press Enter to start Regedit), the registry editor for Windows, please search for the string ";C:".