The "sharing and security" option in the right-click context menu on a folder is no longer there. The File and Print sharing option is "installed" and turned on in the TCP/IP properties, but the ability to share any folders has disappeared, along with all previously shared folders. Where should I check to see what the issue is here?

EDIT From comments below:

"Server" service is not starting, see comment below for more... Firewall is completely disabled, too.

This is XP Pro, Simple File Sharing is turned on. We've discovered that the "Server" Windows service is not starting on bootup for some reason. When attempting manual start, we get "error 2001: specified driver is invalid."