These problems are almost always network related. Double and triple check everything in your network settings. Remove any protcols that you don't need (check with your DSL service provider) and if that doesn't work, wipe out everthing in your network properties and try again.
Chances are the only things you need in your network properties are:
Client for Microsoft Networks;
Your network adaptor; and
TCP/IP for your network card.
You may need others if there are other PCs on your network and you are sharing the DSL connection.