By default Oracle's Virtualbox sets its networking to a NAT mode. In this senario you will have to set it to bridging. Go to the VM in question (in VirtualBox) and edit its settings and set the network mode from NAT to bridging. Note that DHCP will no longer be provided by Virtual Box, but by your standard network DHCP provider if you have one. (If your windows box is set to obtain an IP address automatically then you already have a local DHCP server).
To pick up the new dhcp server's provided address run
ps auxwww and look for a process called
dhclient. Kill this process and then re-run it with the same command arguments. You should then have a new IP address. Restart your name server
/etc/init.d/bind9 restart, and then you should be able to use the DNS server from the IP address provided by
ifconfig. (So long as a firewall is not blocking it).
If your Debian guest has no DNS already, then you will have to install it with
apt-get install bind9 (ran as root).