You say you've tried 'resetting' the modem. Is that a full reset? I.e. shutdown and power off PC. Power off modem. Power on modem, leave till all lights settled down. Power on PC.
You say the problem is not with the Ethernet card, cable or modem because you can connect with your friends lap-top. I'm assuming you have used the same cable, but the same Ethernet card? (Is it a PCMCIA card?)
Any firewall? If so, try switching off temporarily, to eliminate as a cause (just in case the firewall is blocking DHCP requests)
[Edit] beaten to it, with much the same advice.