I had a similar problem. There's a system file (I believe it's a driver) in the Windows folder called ipsec.sys. Norton found a virus in the file and quarantined it, afterwhich I lost my internet connection. After much frustration (System Restore wouldn't work), I gave up and ran a non-destructive System Recovery which re-installed the operating system and returned the computer to its original state when it was purchased without reformatting the hard drive. I lost all my installed programs and windows updates, so I had to download and reinstall Service Pack 3, Norton, Firefox, etc. However, all my data files were preserved including documents, spreadsheets, pictures, music, videos, emails, desktop icons - essentially everything that is preserved with a System Restore.
I suspect there may be a more elegant solution - perhaps if you google ipsec.sys you will come up with something, assuming that file is the cause of your problem.
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