Use a twisted ethernet cable - run that from the DSL box to the Hub.
Then run lines from the hub to the PCs as usual.
Depending on the DSL provider, you can use the connections at the same time, or you may not. For instance, with Verizon, I can have 3 PCs on the hub using the DSL at the same time, but my previous provider, Covad, required a static IP (hence no sharing the connection - but I could share the files on each puter).
But the key thing is the twisted cable. They're fairly cheap on ebay, I picked up a 25' one for $8.
Incidentally, you can network two PCs together with just a twisted ethernet cable, without needing a hub. That's how I have my laptop and desktop networked.
Also, if you do decide to just get another NIC (if the above method doesn't work) two freeware programs that work the whole net sharing thing are SohoConnect and AnalogX Proxy. I can vouch both are excellent programs.