Slow due to unavailable network

I am using Vista Home Premium 64-Bit. There are several other PCs mounted
via LAN. Some with Map Network Drive and some with a Link //192.168.../c$
If all other mounted PCs are up and running, my PC is fast. But if only one
of the mounted PCs is down, the PC becomes verry slow. Even if the pathes to
the other PCs are not really need to be used it seams that Vista try to
connect and wait for a timeout.

What can I do to prevent Vista from accessing networks which are not
available. I don't want to remove and add these connections several times a
day, depending on other users shutting down their PC.

Thank you