I've forgot to untick the "keep me logged in" while logging in the office PC, and right now I'm not able to close session. Is there a way to invalidate all my open sessions and force anyone to log in again?
You can change your password, this should make other sessions not valid. I do not think there was any other way besides using Windows Live Messenger.
To use Windows Live Messenger was suggested by Microsoft as a workaround in http://answers.microsoft.com/en-us/outlook_com/forum/oemail-oapps/loststolen-iphone-how-to-log-out-of-hotmail/f6906d0c-272c-4cc6-a72b-bdfd53195553
Once you are signed in to that mail client, click your name on the upper right corner of the page -- it will show you the number of places you are signed in, then click sign out from everywhere.