If it is like this:
The best way would be to get a WiFi adapter for PC No. 2 because it would be a waste of time trying to get another modem/wifi router all-in-one in the second room to work as a WiFi client. Most likely it doesn't have that function.
Consumer routers, i.e. the TP-Link WR841ND, have that function only when flashed with dd-wrt third party firmware. I have not come across any stock firmware that allows your consumer router to act as a WiFi client, much less a modem/router all-in-one that allows you to do so.
I would go with buying a cheap WiFi adapter instead of ad-hoc between PC. 1 and PC. 2 because Windows Internet Sharing constantly drops my connections and even when the signal is perfect, speedtest.net reports a max download speed which is half of my download speed were the computer connected to a consumer router. The PC acting as a router in my case is running Windows XP SP3 with an Intel WiFi Link 5100 AGN card.