Q: Can you set up airport express 802.11n wirelessly to connect to an existing wireless Internet connection? With a Dell PC?

you have to have the windows airport utility 5.6.1 downloaded here

http://support.apple.com/kb/dl1547

its always easier to plug it in then hit the reset button so it shows up easy in the airport utility you downloaded.

then go through setup and when it asks to set up wifi click the drop down menu and hit extend wifi network, it will then ask you to select the corresponding wifi ssid then enter your wep or wpa passsowrd for that wifi, then hit update and when it reboots you wont even know its there unless your in a rrom that had poor signal then the signal now will relflect the airports extension and have more "bars"

hope this helps

if i understood wrong and you are trying to extend from the dell's wifi it would be a little different process but the same steps need to be taken

this is all based on the assumption it is a Apple network already and the router is set to be able to extend the wireless.

if not you will have to connect to wifi using your pc then share that connection to your AE and either enter in "bridge mode" or create a new network to allow that device to braodcast a Access point seperate from the one you are using to connect via the pc's wifi