Never disable the Timer and Administration Service.
To facilitate this centralized
configuration model, Windows
SharePoint Services 3.0 relies on two
new and enhanced services: the Windows
SharePoint Services Administration
service and the Windows SharePoint
Services Timer service. The Windows
SharePoint Services Timer service acts
as the heartbeat for the server farm
and is responsible for running timer
jobs that propagate configuration
settings across a server farm. The
Windows SharePoint Services
Administration service works hand in
hand with the Windows SharePoint
Services Timer service and is
responsible for carrying out the
actual configuration changes on each
of the servers in your server farm.
The Tracing Service writes to the ULS trace Log. If you want to reduce tracing then do so via central administration (Central Admin / Operations / Diagnostic Logging), don't disable the service.
VSS Writer is used by certain backup applications, you may not need it if you are not backing up your SharePoint environment.
The Search Service is used for Search Indexing. You probably don't want to stop this service, even when you are running another Search Indexer in parallel as it is used for indexing the built-in help.