Here's what I did. Hope it works:
1- Open AirSharing :P
2- Open My Computer, click on "Map Network Drive" (top right)
3- You'll now get the What network folder would you like to map screen. Pick the drive letter you want (I picked I) and in folder type the address AirSharing gives you (I used the bonjour address
http://iPhone.local:8080). Also, unclick Reconnect at logon.
4- Here's where a lot of people go wrong. Before clicking finish, click on Connect using a different user name. Put your login information in the pop up window you get.
5- Once you have your info, click finish.
Hopefully that will do it.