Here’s an ace tip from Channel 10 that explains how to configure Windows Vista to automatically index all .pst (Outlook Personal Storage) files on your computer. This saves you having to open archived .pst files in Outlook every time you need to dig out an old email.
Follow the steps in the tip on Channel 10.
The only enhancement I’d suggest to this tip is that at step 5 you should do this "Quickly type pst on your keyboard to locate the pst entry, then check the checkbox next to pst to include it in the search".
Making this change does force Windows Vista to rebuild your search index and may slow down your PC for a while. Do this just before you can leave your PC idle (before lunch or at the end of the day) if you don’t want to impact your productivity.