One of the most popular questions people are asking is,which services they can disable with vista ...So here's a few that will help with improving your comps performance.

Desktop Window Manager Session Manager - If your not planning on using Aero Glass you can disable this service.

Vista Offline Files - Vista offline files service is used to perform maintenance activities on the Offline Files cache,user logon and logoff events,and dispatches events to those interested in Offline File activities and changes in cache state.

Diagnostic Policy Service - problem detection, troubleshooting and resolution for Windows components Set to Disabled.

Diagnostic Service Host - Same as diagnostic policy service.

Diagnostic System Host - Same as diagnostic service host and diagnostic policy service.

Windows Vista Error Reporting Service - Allows errors to be reported when programs stop working or responding and allows existing solutions to be delivered.

Vista Terminal Services - Allows users to connect to a remote computer. Remote Desktop and Terminal Server are dependent on this service.If you don't use either of these, you can disable this service.

(AresForum)