Windows Vista, XP and Mac network problem

[#1]

Here is my setup
Desktop-Vista
Laptop-XP
Macbook pro-software 10.6.2
Belkin F5D7632-4 Version 1000 ADSL modem router
USB print server set to static ip xxx.xxx.x.2
Before I purchased my mac I had the desktop set to a static ip address of xxx.xxx.x.200 (I have a VPN hence the static ip), the XP laptop was DHCP. If the laptop was switched on it would knock the desktop off the network and then return after a few minutes. I wondered if it was trying to assign an IP to the XP laptop which it already had issued i.e xxx.xxx.x.200 so I changed the DHCP range in the router to between xxx.xxx.x.3 and xxx.xxx.x.9. This didn't seem to solve it. In the end I gave up until I purchased the Mac. Every time it is switched on it does the same as the XP machine and knock the Visa desktop off the network, HOWEVER it continues to knock it off roughly every ten minutes. In order to get my Vista machine to see the Mac I had to google a lot for a solution that involved changing HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Control/Lsa/LmCompatibilityLevel from 3 to 1. I don't know if this is the issue that it keeps getting knocked off but it was the only way my Vista machine would see the Mac. The fact it was gettting knocked off by the XP laptop before I got the Mac suggests it isn't a problem.
The other strange thing is if I am moving a large file from the desktop to Mac the airport just drops out and cannot connect again unless I reboot. It works fine if I do it using a network cable.
As anyone got any suggestions as to why this may be happening as it is very annoying when you are half way through a Skype conversation and it cuts out.
Thanks in advance

[#2]

I have no idea what is going on but that certainly isn't normal behaviour.  I would be tempted to note down the router settings then reset it to factory settings.  See if you can now connect your laptop without the desktop being knocked off the network.  If that works then see what you can do to the router (by way of reverting to your previous settings) to make the problem come back.  If the problem remains after the router is reset to factory settings then the router must surely be faulty.   

[#3]

Reset the router to factory settings but no difference at all. Even if I put the most basic settings in i.e leaving out VPN port forwarding etc it still gets knocked off. I'm going to change the network cables to see if they are at fault but seeing as this issue just appeared overnight I can't think that this is the problem.