Does my PC keep disconnecting from the internet because I'm not on broadband? Is it a devious ploy by suppliers to get you to pay for a "better" service?
I assume you are running Windows, find your dial up connection and right click. Go to properties and click. You should see something, depending on your version of Windows, that indicates when the modem should hang up after a certain period of inactivity. Change this setting to suit your needs. Click OK or Apply as appropriate.
Hope this helps.
Alan,Canada