I used a Netgear router with inbuilt modem, works great. You can get a version to use with an exising router, but for the small amount of difference in cost its best, easier and tidyer to get a combined unit.
You would need a dial up modem which outputs to ethernet WAN port on router and connect from there to the PC and laptop - also via ethernet. Not sure if dial up networking would be able to negotiate the router though. I think you need to take a look round the netgear click here belkin click here and linksys click here sites to see if they actually have such a product.
One way that would work, is to use internet connection sharing and stick a couple of USB wireless dongles in each of the PC and laptop. That would not be via a router though and the PC with the modem would need to be switched on to make the connection which could then be shared.
The nearest I have come across to what you want is a Bluetooth V90 (V92?) dialup modem, which you could use in conjunction with your laptop and a Bluetooth adapter. PC World had Bluetooth modems.
The range would be much less than with 802.11b or 802.11g wireless devices, but these are essentially for Local Area Networks and I haven't heard of WiFi "dialup" modems (which doesn't mean they don't exist!).