From an IT perspective, it's really hard to say exactly where and how this type of thing happens. This kind of thing can often be the result of any number of things such as:
Security risks caused by using an open or otherwise public WiFi access point, or other such shared access connection
A back-door method such as a trojan allowing access to your system
Physical access security risks (not locking your computer, improper boot priorities in your BIOS enabling a UBCD hack, etc)
Improper firewall configuration on your local machine
Malware based browser add-ons or 3rd party software installed on your system
Resetting your browsers and extensions is never a bad idea if you suspect that one or more of your extensions has been compromised. It may also be wise to run a search with SpyBot Search & Destroy (the free edition, it's a little tricky to find) can't hurt things either. It's available from www.safer-networking.org
Is there no way for you to examine my laptop and find out when it was broken into?