If you want to go beyond the basic mailbox search, click the Search box magnifying glass icon in Outlook 2010:
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:
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:
In Outlook 2007, you can quickly search on mails from a particular person by using Tools, Instant Search, Messages from Sender.