AKICILJ/DW: Microsoft Outlook "offline address books"

It just took Microsoft Outlook the better part of two hours to shut down; it said it was decompressing offline address books.

Does anyone know what causes this, and whether it's safe to cancel the process?

I use Office 2003 on Windows XP. (I will eventually screw up my courage and upgrade to Windows 7 and Office 2010 "for the better SharePoint integration", but that's not something I'm really looking forward to, even though I have a new laptop on which to do so.)

Possibly related: This morning before work, I did use the Web Access to the Exchange server, but on the other hand, I'm pretty sure I shut down Outlook before I went to lunch. Is that likely to be related?

Also, please note: I am required by my employer to use MS Office. Suggestions involving using different software will be relatively unhelpful.