Hi Leon,
I'm not sure what the distances are between the two laptops and the PC, but one option is getting a 4g Router for the USB Dongle and connecting the desktop and the old laptop with normal network leads to the router.
That way you dont have to setup anything on the old laptop or PC, they'll just work straight away, and your wife's laptop can connect via wi-fi.
so leave the 4g router near the desktop and the old laptop.
something like this will do the job
http://www.officeworks.com.au/retail...uters/DLDWR113
if you don't want to use cables, just get mini wifi usb adaptors for the laptop and the desktop. just make sure they support windows xp like this one
http://www.officeworks.com.au/retail...ers/NGWNA1000M
That way, all three machines would use wireless to connect to the router and use the 4G.
either way, none of the machines are dependent on each other and can work independently as long as the router is on.
not sure if this suits your needs, but It'll work well.
good luck.