My son uses a WXP laptop with a built in wireless adapter.
My wife's WXP computer is downstairs. She was connected with a wireless adapter for several months until this morning.
This morning the wireless adapter showed a good signal, but never connected to the network. After about 10 seconds it showed no signal for about 3 seconds and then showed the good signal for 10 seconds, ad infinitum for eight hours (while I did virus scans).
My computer and my son's were fine the whole time.
I removed the wireless adapter and connected her up with a powerline/ethernet adapter. She was fine then.
I then put her wireless adapter on my computer and disconnected my ethernet cable. I got an excellent and solid connection over her wireless adapter.
So I have shown her computer and the wireless adapter work normally (as well as the wireless router).
What is left that could be a problem?
While the powerline/ethernet adapter is working okay, it means leaving stuff on when the computers are off, and several extra cables in awkward places; I don't like it.