I believe the current version of Keyman is no longer free (~ $23 for the basic version). I got a new computer last spring and had the same problem -- I needed to configure it to type non-Latin alphabets -- and ended up deciding that it made sense to pay for the program. The price seems reasonable and it's important to me to have the functionality.
Until recently there was a page or two (I don't remember where, and I can't find it again) offering an earlier (free) version of Keyman for download along with a Greek keyboard, but I suspect that the program may no longer work properly with current versions of Windows if you do manage to find and install it.Sibylla
may be another alternative.
Windows does have a polytonic Greek keyboard, although it's nowhere near a user-friendly or intuitive as the Keyman ones. Or you might take a look at this
. If you're willing to put some time into it, I think it's also possible to modify a Windows keyboard to create a custom version.