One user on network losing internet connection abruptly

Imagine the following situation; you are an IT administrator with more than 500 end users. All users are running on either Windows 2000 or XP. The internet connection for one of your users starts dropping abruptly with no reason at all. On restarting their computer everything is back to normal only for a short while but then the previous situation occurs again although the AOL IM service runs well even when unable to access his mail. On running net stat you detect that there are unidentified open connections being used to certain ports. This user works from home most of the time using a laptop and you doubt whether someone is trying to access his computer illegally. You are not certain if this user has all the updates installed properly. What will you do to correct the situation and prevent a repeat from happening? Please help for I am stuck in this situation.