What's the hardware config for the Win64 machine?
Check for driver updates.
You know, I occasionally get this as well, and I reset my dns to use the open google servers long ago.
So far every time I've attributed it to a noticeable problem with my DSL connection, usually just a temporary down. As an example, the PPP server seemed to go offline for a while the other night. Everything else was working, but no PPP connection, and no internet ip address, thus no Internetz (tm)
BUT. I used to have more troubles with it, and thus long ago swapped to using googles servers, and it made the problems shrink quite a bit. I attributed it to a weak DNS setup via AT&T at the time... (att runs a dns server which feeds all the modems, the modems feed any home routers installed, etc, in theory. The conclusion I came to at the time was the modem wasn't always picking up the dns properly and propogating it to the router)
For the record my box is running a realtek onboard nic, with the latest drivers at the time I updated them, which was some time ago.
Do you have other Win64 boxes on your network, or is this the only one?