I had an issue on a SharePoint 2007 server recently that wasn’t immediately obvious to me (it may well have been obvious to you however).
Whenever searching using the Microsoft Search product the following error would be returned immediately:
The search request was unable to connect to the Search Service
After a little bit of head scratching, checking options in Shared Services and checking of Application Poole accounts I was looking at the list of services in Windows Service Manager and noticed that the Office SharePoint Server Search service had stopped (unbeknown to me). Restating this service fixed the issue immediately.