Strange DNS problem

The basic setup at my house is a DSL modem connected by USB to my flatmate's computer (it's the more convenient one to the phone jack), then WinXP's built-in internet sharing giving me net access on this machine.

It's been working fine for around a year, but just recently I've had major problems with resolving domain names on my box, while the net continues to work fine on my flatmate's. A reset of my machine does nothing, but a reset of his fixes the problem for a couple of hours before it occurs again.

I've tried restarting the DNS service on his machine when the problem happens, but no luck. We both run ZoneAlarm Pro but neither of us have made any changes to the settings, so a firewall problem seems unlikely.

Can anyone possibly come up with some suggestions as to what might be wrong? For reference, my flatmate's box is running WinXP SP2 and I'm running SP1.

If restarting your flatmates computer fixes the problem (albeit temporarily) then the issue is with his machine, I would say. Change your DNS. Yes, I know it's probably assigned by DHCP, but just ipconfig, get your ip and subnet (192.168.0.2, 255.255.255.0) and use your ISPs DNS directly, or find a free one on the interweb to use. If problems persist, it smells like a firewall issue on your flatmates computer. That, or spyware. Mind you, if he's connecting the modem to his machine via USB, there's no way in hell I'd trust him with my internets. I suggest you set it up via ethernet. USB adds random problems (like this one) so it could be what's at fault.

Actually, buy a router.