System error with dsl connected

Hi,
I'm having a problem with a desk top system connecting to a modem. I don't think it is the modem because it works with a laptop in another location. I've tried two different brands of network cards in the system with the same results. When I connect the ADSL modem to the computer I get an error message:

D:\\WINNT\system32\lsass.exe terminated unexpectedly with status code 128. The system will now shut down and restart.

I cannot just turn the system off, the i/o switch on the back must be used to shut down the machine. If it reboots it will come back to the same message and routine.

If I disconnect the modem cable the system works normally. I read that the message has something to do with a worm but if that is the case why would it only appear when a modem is connected.

I did a virus scan with Nortan, managed to download their updates before the message appeared and shut down the system, but that didn't help.

The working system is a laptop / Win XP home on the second floor. It is currently connected to the ADSL moded thru a port replicator and works properly.

Does anyone have any idea why the modem will work at one location but not at the other?

I tried the sasser worm fix but got a "no virus found" message so I don't think that the worm is causing this problem.

Thanks, TH