That depends. Without it on , you have to be sure to use the old-style Search Assistant for your searches. Search Indexer builds a lovely index of all the files on your system, making searches faster. But when it is installed, your default search uses the index database. If that database is stale, it can't find things.
So you have to use the old-style Search Assistant, which is an option offered in the bottom left of the search pane.
Two options. First, remove the search indexer from the system, and the assistant will now be your default search. Second, tweak the options in Search Indexer to make it less aggressive. I don't know how off the top of my head, but I'll take a look.
Anyone else have runaway indexer problems that they solved?
EDIT: Peek through this, which addresses a couple of possible causes: http://social.msdn.microsoft.com/Forums/en-US/windowsdesktopsearchhelp/thread/163713af-59dd-44fe-b13e-dcca7939e8b4
Or Google "search indexer high cpu"