We are having similar problems. We have 3 new Win2008 servers that we are trying to put into production. We've been testing everything out on one that is dedicated to development, and it has been running fine for 2 mos up until yesterday. One of the apps we were in final stages of testing was trying to open an Excel spreadsheet and throwing an exception 'Microsoft.Jet.OLEDB.4.0' provider is not registered on the local machine'. Google found links that 32-bit apps needed to be enabled for the appPool. As soon as we did this then the appPool shut down giving the 503 Service Unavailable error, with the following events
AppEvent 2280 RpcProxy.dll failed to load.
SysEvent 5139 Listener channel failure
within hours all other websites on the dev server also shut down! At this point we are completely down, as this has impacted our bug tracking, source code control, CMS site, etc. Turning 32-bit off doesn't help. Tried restarting appPools, WAS, webservice all without resolution. ProcMon shows a couple of dozen Name Not Found results, but nothing definitive. IIS is running under NetworkService