Dell D620 uses Broadcom BCM43xx based wireless network card which may not work when you try to install Ubuntu. There are already drivers for them.
1. Check the following page for identifying your chip set, available drivers in Ubuntu, and how to install the proper one with or without network.
2. Once the driver is installed, you may need to manually bring the wireless card up.
sudo ifconfig wlan0 down
sudo ifconfig wlan0 up
3. scan your network to see if you can see your router!!
sudo iwlist wlan0 scan
4. Feed the ESSID of the router found in the above scan into the following.
sudo iwconfig wlan0 essid "ESSID_IN_QUOTES"
sudo iwconfig wlan0 mode Managed
sudo dhclient wlan0 &
5. May have to connect to the router using network manager by hand.
6. Reboot may help.
Also see: Dell D620 Wireless problems