Problem with switching between LAN and dialup connection on laptops

For some years i have had this problem on diffrent
machines and also diffrent versions of windows. Now i
have dcided to see if someone knwos how to solve this
problem forever.

The case goes like this.

On a laptop i have XP installed. I let the NIC be
installed as default - that is ig gets configured
automatically via a DHCP-server. (Most LANs today are
useing som NAT technique and DHCP so the this
configuration makes the laptop easy to move around to
diffreent locations). I connect the computer to an LAN
and its works fine.

After this i configure a dialup connection for browseing,
I unplug the LAN and test the dialup everyting works
fine. I disconnect the dialup connection.

Now I plug in the laptop into the LAN again and opens a
Internet Explorer browser window. Some times it will work
fine, often it will not. What i get is the little window
asking me if I want to connect to the Internet via the
dialup connection or if I want to stay ofline. Obviously
the computer is connected to the internet via the LAN but
XP does not understand this. It dosen't matter how I set
the Inernet options in Internet Explorer on how Explorer
are going to use the dialup connection. After some fixing
and some resarting the problem often diapears, but when
you swtich between using modem and LAN to connect to the
Internet often it realy gets booring.

Is there anyone hwo know how to fix this problem.