Gmail – Account Accessed from Microsoft’s IP Address, Hacked?

If you used the same e-mail password as for another site, it is possible that that site was compromised, and they obtained your password that way. But it's still worth running a full virus scan on your computer anyway. Most virus scanners can detect keyloggers these days (the most likely form of attack).

You could be in one of the following situations:

• you could have a keylogger/trojan on your computer;
• you could have logged in from a computer that had a keylogger/trojan installed;
• you could have been a phishing scam victim;
• you could have used a service that required your gmail account password (less likely) and they either store it in plain text and got hacked or were just rogue and sold/used it.

Now, you should:

• change all the passwords that you received on your gmail account, for each service/server;
• get in touch with any financial organization (e.g. banks) that might have sent you sensitive info on this email - if that's the case, follow bank account and credit card movements.

And last, you should switch to 2-step verification for your gmail account. Details are on gmail blog: http://googleblog.blogspot.com/2011/02/advanced-sign-in-security-for-your.html

Also, make sure your computer is clean - no viruses, trojans, keyloggers.

Edit:

It's also possible that you have the password stored in plain text and that file got somehow misplaced (trojan, file sharing error).