I have XP-Pro on VirtualBox on a Windows 7 Pro machine.The Win 7 machine is running Avast AV and Firewall. Suppose while surfing on XP I download a virus and the XP machine is infected. Will this be detected and dealt with when I scan theWindows 7 machine?

Generally speaking, a virtual guest machine infected with a virus won't be detected by the anti-virus running on the host computer. Anti-virus software operates by scanning files that are transferred in and out of a computer in real time, as well as detecting viruses that attempt to run in memory as executables. The hard drive and memory of the guest Windows XP virtual machine is effectively invisible to the Windows 7 computer, and therefore is also invisible to the anti-virus scanners in the Windows 7 host computer.