In addition to the “Trouble upgrading your cluster to Exchange 2003 SP1?” steps posted to the blog a few weeks ago, here's another error I've seen posted on the newsgroups recently and a few common solutions:
The network path was not found. Facility: Win32 ID no : c0070035 Microsoft Exchange Cluster Administrator Extension
Generally we see this when trying to create (or upgrade) the System Attendant resource on Exchange 2003.
Here's two things to check, either of which has been known to cause the network failure above: