Hmmm, when setting up my uni network - NTL dont support connection sharing, so I suppose moost ISPs dont. If the two are directly connected, then they'll share an IP, which is bad - as it will conflict, as you say. If you use a network switch (like a router) you plug internet into that, it assigns virtual IPs to all PCs on the network (that are also connected to switch / router) and then you run the Internet connection wizard - connecting through a 'residential gateway'.
That should work. Or, a solution I've found (certainly with NTL) is plug the Modem (if broadband of course) into a USB port, then set up the IP software if required. Then that PC has internet. Then use a CAT5 network cable to go into a router from your network card, and the otheer pc iinto the router via CAT5 cable. Run setup wizard, using the XP disk, selecting acccesss internet througgh another PC. Again, this fixes the IP isssues, as it assigns a virtual IP.
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She says brief things, her love’s a pony
My love’s subliminal