IP conflict when double sharing Internet

Hi, I need some assistance:
We have two computers: a desktop and a laptop and we get Internet with a 3G modem plugged into the desktop and then an Ethernet cable goes from it to the laptop.
I also have a tablet PC and I need WiFi for it.
Originally the desktop had Windows XP and the laptop Windows 7. I was able to get internet from the desktop to the laptop and then in the laptop I was able to share that shared connection via Wifi as well :). But we've upgraded the desktop to Windows 7 and now I can't do it as it's giving me an IP conflict error (because when turning on sharing windows 7 sets the IP address to 192.168.137.1 (or something like that) and when I try to enable sharing again in my laptop it "steals" this IP address and the conflict occurs) and I can't manually change the IP addresses.
Do you have any tips on how to set this up? I hope you understand what I'm trying to achieve . In short: we only have one source for Internet - the 3G modem but we need to spread it over 3 PCs: from desktop to laptop via Ethernet and from laptop to the tablet via WiFi. And have it work at the same time.
Thanks

Hi. So the desktop is the primary Internet source because it has the 3G modem plugged in and if I disable ICS on it there's no Internet connection on the laptop and therefore nothing to be shared from the laptop to the tablet via WiFi. So I'm still stuck...

You have a router or a USB Stick ?
Maybe country is not established

Yes, it's a USB modem

Drop the Modem and buy a 3G router with an external aerial. Phone your Carrier's Customer Service and ask to be put through to technical support & ask them for the exact settings that the 3G Router is asking you for. I did this in the UK as I couldn't get 3G on my phone on holiday but the laptop got 3G on the dongle, bought a 3G router with an external aerial & stuck it up a pole, got a few clicks of 3G which was enough for the laptop & my phone (just). This then solves all your problems of IP address conflicts & having to connect the desktop to the laptop with an Ethernet cable then create a portable hotspot on the laptop for the tablet.

I'm not sure why I would need to be purchasing anything additionally if previously I was able to set this up when the desktop had Windows XP...

I've tried the virtual router software and once it creates the Wifi network it has no Internet connectivity

check the country I've said that before ! in 192.168.1.1 watch if your country name is visible and set it to yours !