View the source of your webpage from a Windows machine and check thee CSS to make sure the Google font is being properly embedded. If you have the font installed on your Mac it will display properly regardless of whether you've embedded it into the page properly.
The more likely explanation though, is poor anti-aliasing in the version of Windows you're using. I'm not sure where these settings are in other version, but in XP, right click on the desktop, select "Properties", go to the "Appearance" tab, then click on "Effects".
"Use the following method to smooth edges of screen fonts" should be checked, and ClearType should be selected from the drop down list. This will give you font anti-aliasing in Windows that is as close as possible to what you'd see on a Mac.
In XP the default was the "Standard" anti-aliasing, which did very little, but in newer versions I believe "Clear Type" is the default. I'm not 100% sure, but if you're going to have many users with a default XP setup and this turns out to be the source of the problem, it might be a good idea to reconsider using this font.