Modem???? Not to sound silly, but aren't there any other options to you in order to connect to the internet? If not then I suggest upgrading to a modem that is currently on the HCL for windows 7. I have not looked for one myself, so I am unaware what might be on there. Let's look..
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Might be able to get another card for your machine or better yet, look into a service provider in your area that offers HS Internet connectivity. Since your machine has a modem, chances are it does not have a nic. You might want to look for HS internet and then get yourself a NIC instead.
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Good Luck!