Hmm. Maybe you need to start looking at the programs that are running on your PC (both wired PC's). And if this doesn't help, formatting both PC's (or just the one your girlfriend doesn't use) and seeing if it works without any programs installed.
Failing that, then it's definately a modem problem...unless you are able to try your external HDD on someone else's internet with a different modem. Obviously if it does happen there, it's the external HDD.
09/10/2007
Mario and Sonic at the Olympic Games
Gold Coast, Queensland, Australia