If you want to go beyond the basic mailbox search, click the Search box magnifying glass icon in Outlook 2010:

Search box magnifying glass icon

This opens up a Search Tools toolbar with a number of options. Let's say you want to look at all mail from a particular person, even if it's filed away outside of your Inbox or selected mail folder. Click All Mail Items and then From and type the Sender Name:

Search toolbar

If you want to narrow things down further, you can query just mails with attachments, flags, marked as important, etc. Or click Search Tools and select Advanced Find if you prefer a more traditional dialog box:

Advanced Find dialog box

In Outlook 2007, you can quickly search on mails from a particular person by using Tools, Instant Search, Messages from Sender.

Suzanne