Intermittent Problem

I have a network with three WinXP computers. One wired desktop and two
wireless laptops that connect to the internet via a cable modem
connected to a router. Everything works just fine except, at times,
but not all of the time, the laptops cannot access files on the
desktop, or even access the network. I get a message that says they
are not authorized to access the network. (They can still access the
Internet.)

Any suggestions will be appreciated.

Regards, jimbo