Hey there Alfred,
Now thats an issue which might have many causes.
Firstly , Do you have Wireless router setup on the computer ??
IF you do or even if you dont, you might have Client For Netware ..
You can find out by following the steps:
Go to Control Panel and then to Network Connections,
then right click on any of the LAN icons and goto properties,
Under which in Installed Protocols you might find Client For Netware ..
Now, this Client by default disables the welcome screen and no matter what you do ..you cant get that back until you uninstall the same
But Beware: uninstalling that might disable your Wireless capability...or your Internet Connectivity