Internal ADSL modems are few and far between now.
I really would consider a router, even if you only have the one PC. They don't take up any more space than a modem. Routers connect via the network or ethernet port on your PC (if you have one) and as ethernet is made for networking unlike USB you get a more reliable connection and there are no drivers to install etc. It's practically plug and play.
This B-grade one would do you fine click here I got one from then and it's excellent, like new.