Windows keeps forgetting other machines

We have 4 windows machines, ADSL-modem with 4 port router and a WLAN base station connected to one of the ADSL-box ports. The WLAN-box is set to hot-spot only.

One of tha mechines is a desktop running Windows 2000 and connected directly to the ADSL-box. Another desktop is running Vista and is connected via the WLAN. One machine is a laptop running Vista and connected via the WLAN and another laptop running Windows 7 Basic is connected via the WLAN.

The old Win 2000 machine is mostly used as a network disk and printer server.

Now the problem is that the laptops keep forgetting the other machines in the LAN.
Printing fails and trying to access the Windows 2000 machine's disks fail.
After rebooting a laptop and re-configuring the network printer everything works
for a while, but after some time, some machines are not found in the network.

Writing "\\OLDPC" on the top line of the file manager finds the machine for some time
after the "OLDPC" has disappeared from the "network", but after an hour or so, that doesn't work any more. Rebooting the laptop makes all work again - for a while.

It's gettin very annoying.

Any ideas what could be causing that?

[edit]
Oh, forgot, The problem does not affect internet. Internet works all the time.
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