Stephan,
The best way I can think of to solve this problem is to use the Windows API VIs for LabVIEW. They are available in the NI Developer Zone under the name "Windows API Function Utilities (32-bit) for LabVIEW". Specifically, there is a VI in the winevent.llb called "Get Task List" that returns the names of all the tasks currently running in the toolbar. You can just search each window name in a for loop for "Microsoft Excel" and if you find it, you'll know that Excel is running.
I hope this suggestion gets you started in the right direction. Good luck.
Darren