strange things happening to my computers

[cjm]

relativly new to this limewire thing ,I recently got a broadband connection & connected 2 macs through a router all with standard setings ,I had a major crash on my ibook with all my mail,itunes address book and desktop files showing up in a folder when I re booted , I reloaded the applications,but since that day I get a weird message when I open my mail application (apples own) I am getting a bit paranoid as someone told me people can "Hijack" your computer from ftp sites is this true ?if so what do I need to do?

Please Help