Recently, we have seen some performance problems on Exchange servers being caused by "desktop search" products that plug into the Outlook client and then index mailboxes and/or public folders for faster searching.
This does not mean that all desktop search products index mailboxes and/or public folders and all exchange performance problems will be related to Desktop search engines.

Couple of related points that we need to look into are:

• The huge number of RPC requests that those clients could start sending to the server might cause a client-side bottleneck.
• The Exchange server gets hit with substantial requests while applications are crawling mailboxes, increasing the IOs per user, and creating a bottleneck in the disk subsystem.

Following KB article will elaborate on the subject, as it contains few steps for addressing those issues, as well as troubleshooting tips that can help you identify those problems. Read more from the article: http://support.microsoft.com/?id=905184