I was having a similar problem. I was connected to the network but not the Internet. All of my XP machines were getting internet without a problem.
I did two things and the problem seems to be corrected.
1. Upgraded my firmware on my router (not sure if this did anything but it seemed to be dropping the internet connection less than it was previously)
2. Turned off IP6 and both Link Layer Discoveries in the LAN Connection Properties. This got me back online immediately and I haven't had a problem since I've done this (we'll see if by next week it is still running smooth)
At least something to try!
Sarah
