Upgraded users to Win7. Now getting "path not found" when saving files or opening attachments

I had a similar experience. I know you mentioned the only change being the introduction of win 7, but I'd check beyond just that. In a previous job, our team had a number of servers in a cluster acting as file shares. These contained DFS and other folders that were targets from redirection. When one of the servers had a NIC issue, we used the downtime to update drivers. This was a bad idea, because the driver update had changed default settings and turned on TOE in the NIC Teaming settings. TOE or TCP Offload Engine usually optimizes Network by several techniques. One of these was remembering network routes that had changed. All of a sudden we had users calling with similar issues to this one... Not much though, only a few a day. When we finally tracked the problem down, simplying turning off TOE solved it for us.

Edit: forgot to add that I'd need proof to be convinced both those issues are related.