So this is one of the strangest issues I've seen...
I have a Windows 2000 Server box with SP4 plus all recent Windows updates. I'm not sure when this started (the server is a backup and isn't often used), but after being on from anywhere between several hours to a day or two, the server won't be able to make any outbound network connections. Loading Internet Explorer just results in the "This page cannot be displayed" error, trying to browse to a network resource returns "The path was not found", if we try to connect via PCAnywhere to another machine on the network, the connection times out. However, if any inbound connections are made to the server, such as remotely controlling the server via PCAnywhere or browsing to a shared network resource hosted on this machine, all is fine. Once the machine is rebooted, the problem clears up for a few hours to a day, then suddenly starts up again.
As for event log entries, the only one I've seen is an entry logged by the browser service saying it cannot retrieve a list of browsers from the browser master.
My first thought was that this is a virus or malware. After performing several scans with multiple Antivirus and anti-malware apps, no malicious software has been found. I've also swapped out the NIC. This hasn't fixed the box either. The network firewall isn't blocking the server and IPSec is not enabled on this machine.
I would just rebuild the box but it contains software that only our vendor can supply and they want a very large fee to reinstall it. Are there any other options that exist?