I use broadband cable all the time, but occasionally I need to send an email using my dialup ISP.
I have the dialup network, (DUN), icon for my dialup ISP on my desktop and just click to open then click connect, and the modem dials up the ISP. I don't have to change any settings in IE or anywhere else. IE is always set to, ''Never dial a connection'', but I can still browse and surf using it on dialup.
You should note, whilst using dialup you will not get broadband speeds. However I haven't tried Bob's suggestion so I don't know if Netswitcher allows you to use both at the same time, eg use one browser for dialup surfing and another for cable speed surfing at the same time.
Mark