It is completely possible to network them and share your ADSL connection. Things you need to make sure you need besides the crossover/twisted pair cable: 1 Network Interface Card (NIC) for each machine (no router needed). Your will connect the NICs, again one in each machine, to each other with the crossover cable (NIC to NIC).

a) If you are using the default XP "Category View" in the Control Panel click on "Network and Internet Connections" then click on "Network Connections".
b) If you are using the "Classic" view in Control Panel click on "Network Connections"

2) You should now be in the "Network Connections" folder. Right click the network adapter connected to your ADSL Modem thru USB (probably named "Local Area Connection 1" --very important you choose the correct one, the one hooked up to the ADSL modem) and select Properties. Once done, click on the Advanced tab. This is where you will have the option to share the internet connection.

3) Put a checkmark in the box that says, "Allow other Network users to connect thru this computer's internet connection". You should be done with the Internet sharing portion.

You should be able to access the internet from both computers. Sharing any files and folders is another story and I will not get into it in any detail as you did not mention it specifically. If you have any trouble with the above instructions, please see this Microsoft KB Article: http://support.microsoft.com/?kbid=306126 and for setting up shared files/folders and drives: http://www.microsoft.com