In the Virtual PC Control Panel, highlight the guest OS (in your case Ubuntu) and click the Settings button. In the dialog that appears, click on the Networking setting on the left side. On the right the dialog will update to show you network adapters. In the drop down for Adapter 1, change it from your current network card to the Dell Wireless card, then click OK. That should fix it, although if you have Ubuntu running at the time you change the setting you may have to restart it.