I'm guessing a bit here because I use an English keyboard and I'm really not familiar with the key layouts for other languages. But I suspect you can do this by setting the keyboard to a language in which the "," is the decimal separator. You can do this as follows:
Start -> Type "Region" in the Search box -> Click "Region and Language"
Click the "Keyboards and Languages" tab.
Click the "Change Keyboards" button
Change the "Default input language" to the language you use. If the language doesn't appear in the drop-down list, then click the "Add..." button to add it.
thanks but this method doesn't work for the Swiss/French keyboard