Q: What’s more annoying than reloading your operating system?
A: Getting Outlook to relearn who you send emails to.

When composing a new message, as you key the recipient into the “To” field, Outlook will automatically attempt to find & match the intented recipent with a history of previously sent messages. This history is stored in a nicknames file. In Outlook 2003, the nicknames file is located in c:\Documents and Settings\ %profile%\ Application Data\ Microsoft\ Outlook\ Outlook.nk2

Finding it might be easier with Windows search, though. I’ve seen this file grow to a very significant size in some cases. Guard it with your life, unless you don’t mind reteaching Outlook your recipent list for another 6 months or more. You’ll be glad you did when it finally corrupts itself.