Invalid IP?

My sister has been having problems with her dsl connection, and I am wondering what is the problem. She gets a good connection, but then all of a sudden she loses connection. The only way she says she can get it back is to shut down, unplug the power and ethernet wires to the modem for a few mins, and then power back up. She is running XP pro. To me it sounds like an isp problem, and when she called they say that her ip is invalid and she needs to call the manufacturer of the computer (me). She can get on though, so it seems weird that her computer would all of a sudden change the ip it uses. I am waiting for her to run spybot and norton av, but does anyone else have any ideas?

Is there a certain time it allways takes before the connection cuts off? Usually stuff like that is due to the firewall blocking a dhcp request or something similar.

When the computer starts it will request for an ip, and get one for a set amount of time. After this lease time goes out it has to say it wants to keep it. If your firewall has blocked some strange processes one of them might be the one that would announce you want to keep the ip...

If it would be your isp the connection light would blink on a modem when it disconnects. Just look at the modem next time when it disconnects.

There also might be a program conflict, like norton with something or norton by iteself.

The Ip address does it start with something like 169.....