Recently, on computers running Windows Vista, and occasionally on computers running Windows XP, you might start the process of copying a relatively large file from one computer to the other. The copy process starts out smoothly (ruling out complex issues like name resolution, or permissions, or even visibility).
Well into the copy process, with several Megs of file content copied, the process abruptly terminates with a monolithic message
The network location is no longer available
Well, what now? Did the other computer go off the network?
So, you start layered diagnostics.
And, you find no problem, with either the copy source, or target. Maybe you try copying in the other direction, or start the copy from the other computer, and sometimes this will make a difference.
Frequently, the cause of this problem will be simple. With Windows Vista having been on the market for a rather brief amount of time, the vendors of the various networking adapters are still developing drivers for their products. Either the vendor of the network adapter in your Vista computer has not produced a driver specifically written for Vista, or the driver produced has not been sufficiently tested.
So now, you contact the vendor, and ask about a newer driver. But go to the vendor, not to Microsoft.