The best answer to this question is to put wyUpdate.exe and the client.wyc in your "Program Files\ProductName" folder and not an "Update" subfolder. Is there any reason you're not doing this?
If you really must use a subfolder then you'll have to pass the "base directory" (i.e. where your app files actually are) as a commandline paramerter when you start wyUpdate:
wyUpdate.exe -basedir="C:\Program Files\[Product Name]"
Tell me if this helps.