It's probably two computer using the same name. Duplicate name and IP address are unwanted in Windows networking. Simply change the name of one of the computer to something else:
Right click on "My Computer", select Properties, Computer name tab, click on the change button. After changing it, you will have to restart the computer. Then on both computer, go to Start Menu, select RUN, type in cmd. Type nbtstat -R and you should be good.