Receiving replys to emails I did not send, from people I do not know.

Dec 24th: sending a few short holiday messages. Noticed outbox not sending emails. Rebooted laptop. Uploading Windows Lived, a message box popped up saying unusual activity on my account. Wanted me to check OK to open box. Another box popped up asking me to allow Microsoft to make changes to my computer. Also had to sign back in (1) into Microsoft (2) into Hotmail (3) into Windows Live (4) into Windows Live Messenger. Received a new PW by text on my cell. Started over signing in beginning this time with Hotmail, then on and on. 68 new emails in my inbox...  replies to inquiries on accounts, messages to and from people I do not know. Sent all msgs to delete file, but still on windows live. Help. What happened? Hiow do I keep it from happening again?