I have a Windows Server 2003 server running NPM 8.0.78 on IIS 6. I was having a problem with the web interface, where the account used for 24/7 monitoring would be logged out and asked to re-authenticate almost daily. From searching this forum it seems I'm not alone.
I spent a few weeks on and off trying to figure out what was going on. Finally, after pouring over the event log one day I found the cause: by default, IIS recycles its child processes every 1740 minutes (29 hours).
Leave it to M$ to set such an awkward expiration time. A quick search led me to this thread: www.thescripts.com/.../thread307020.html
To disable IIS process recycling, open the IIS Manager and navigate to [server] -> Application Pools -> DefaultAppPool. Right-click DefaultAppPool and select Properties. Uncheck "Recycle worker processes". Those with high-load servers will probably want to enable some other form of process recycling to avoid gradual memory consumption.
The web interface hasn't had to be re-authenticated since implementing this change. Hopefully someone else can benefit from my grief. 
