You don't provide the following information:
Version of Outlook
Type of mail account
However, failing that information my first guess is that you have Google
Destop Search installed which has this known behaviour. Either remove the
Outlook integration, uninstall GDS, or upgrade it.

Signature
Kathleen Orland - MVP Outlook
Outlook Tips: http://www.outlook-tips.net/
http://www.howto-outlook.com/
> see subject