From http://www.c3b.co.uk/?p=16

1. Close Outlook
2. Move (or copy and delete the original at the end if you’re paranoid like me) the outlook.ost file from it’s current location (usually c:\documents and settings\{user}\Local Settings\Application Data\Microsoft\Outlook\outlook.ost) to a new location
3. Open the mail settings applet either from the control panel or by right clicking on your Outlook link and selecting properties.
4. Click E-mail Accounts…
5. View or change and existing account and click next
6. Highlight your exchange email account and click Change…
7. Click More settings…
8. Uncheck the Use Cached Exchange Mode and click apply
9. Click Offline Folder File Settings…
10. Click Disable Offline Use and agree with the pop up dialog message.
11. This will close the Offline Folder File Settings dialog so click the button again
12. You’ll find this time that the File: text box can now be edited. Click browse and locate the outlook.ost in its new location and click OK.
13. Check the Use Cached Exchange Mode again and click OK.
14. Click next and then finish.
15. Close the mail settings applet and launch Outlook again.