E.g. I install Ubuntu as my main system without anything else but VirtualBox and maybe some programs that need to be installed to run VirtualBox.
Then I install within VirtualBox Windows 7 (because I need to run a custom program from my comapny that is exclusively for Win) and maybe another Ubuntu or some other Linux distro for my daily stuff (browsing the web, testing some "untrusted" linux apps, downloading stuff via torrents etc.).
The question is, am I safe when I run apps/browser within my VirtualBox or do some viruses/malware/spyware exist that can access my main Ubuntu system from Windows 7 or the other linux distro that runs inside VirtualBox?