The long running joke about instantly getting a performance boost inside Vista, especially when it is a guest VM, is to do net stop wsearch.
The next best thing you can do, besides limiting what folders are indexed, is to update your Windows Search to version 4.0. Most of the changes between the version included in Vista RTM, and 4.0, revolve around performance enhancements. It becomes especially noticeable on systems with limited I/O bandwidth (think laptops with 4200 rpm HDs).
However, this still isn't perfect. I've talked quite a bit with the Search team about this at the MVP Summit earlier in the year. There is a tweak I do on every system that runs Vista/Windows 7 or Server 2008 with the search/indexing engine enabled. I make sure to use Windows Search 4.0, for the performance improvements and the additional GPO/registry settings available.
The tweak? Disable the indexer backoff. You probably didn't realize you could even do this, but it is an available option that tells the indexing engine, "Don't worry about system activity - just index and get it over with already!". I'd much rather suffer with high CPU / I/O usage for a little while than a long and drawn out 'trickle' of activity that gives Vista a bad name.
This is particularly noticeable on a new install of Xobni when it indexes your Exchange mailbox and touches almost all your mailbox contents. With the indexer backoff enabled, which is the default on all operating systems, Outlook performance drags for a long time. With the indexer backoff disabled, the SearchIndexer.exe process will kick into high gear, finish and fall back asleep. Perfect.
How do you disable the indexer backoff?
For use in a group policy object (GPO):
Assuming you have the Windows Search 4.0 .ADM template added to your domain, or are on a Windows 7 system, look under Computer Configuration\Windows Settings\Administrative Templates\Windows Components\Search. The setting to disable the indexer backoff is not surprisingly called Disable indexer backoff. By default, it is set to Not Configured but you will want to set this to Enabled.