When you're in Google.co.in, here should be a link toward the bottom right on the Google main search screen that allows you to switch to Google.com before performing a search. (At least, I see that link when I'm logged in to my Google account and open any of the regional search engines.)
I also came across this article on how to set your default search to Google.com when using Google Chrome as your browser:
(You may be able to extrapolate those instructions to help with other browsers such as Firefox.)
Hope this helps!