Microsoft has provided several search tools in recent versions of Windows, and it's been a fairly confusing journey. However, Windows Search 4.0 is the current iteration of what Microsoft would call a "desktop" search service, and Search Server 2008 (either full or express) is Microsoft's "enterprise" search service (assuming you're not using SharePoint). Windows Search 4.0 may be enough horsepower for many small and medium-sized organizations, and the free Search Server 2008 Express Edition may be attractive for firms that need the power of enterprise-class searching without the cost.