It is quite unlikely that the hack was *into* your computer. Rather, the PayPal account was likely hacked by someone guessing your information or your information was intercepted via a phishing attempt. That said, I'm sorry that it happened to you. If you believe that OneCare allowed access to your PC through the firewall or some other hacking attempt, please contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator