Just come back from seeing the customer where I installed the very same D-Link kit mentioned above - "suddenly stopped working cust said.".
What do you know, "someone" switched on the windows firewall on top of Norton Internet Security bring the whole shabbang to a halt. A lot of finger pointing, no one came forward so we balamed it on BT - everybody happy.
The symptoms were as you described Captain Over - on/off constantly on the wireless PC - in addition no web service on the hard-wired PC - router seemed to be happily churning away it's network messages and probably wondering "where is everybody?"!!
amowf