I'm not quite sure how "lightweight" you want, but I've had an extremely pleasant experience with Xobni. It integrates directly into the Outlook UI. In addition to providing extremely fast search, it also provides a bunch of other benefits, such as contact information extraction (automatically figuring out people's phone numbers, for example), showing an entire thread of discussion inline à la Gmail, and optionally integrating with LinkedIn. Because it integrates just with Outlook, rather than running all over the system like Google Desktop, the load on my system appears to be roughly the same as it used to be with LookOut; your mileage may vary.