Welcome to the forums :emotion-21:

What system do you have?

Whatever it is... try reinstalling/updating the LAN drivers :emotion-55: 

You should have a drivers disc, but you can get the latest drivers from the downloads page for your specific system, on the Dell website :emotion-5:

Thanks for Tips ...   my System: Studio XPS 435 MT, Win7 64 Bit Home Premium, Intel I7 950.

so crazy: firsttime this morning after System-start: the LAN-Icon has no yello exclamation-mark!  i hope, it will stay ...

ok, 3 hours later .....   the same situation: Network-icon with yello mark ..... now deaktivate and aktivate and now ready for working ....
when i look for newer Driver ....  the install says: there is the newest Driver installed ....