Outlook 2003 Crashing when running in Cached mode

HELP!!

When Outlook is being fired it keeps on saying it is “Retreiving mailbox data of USER****” this process takes sometime, infact today to well over 40 minutes on a 6.5GB mailbox. Outlook is running in cached mode and have unchecked send & receive define groups but still taking a long time to recover data. This is happening for around 40 peolpe in my firm, mailbox sizes differ from largest 10GB to 3.5GB.
Offten happening when PC is fired up in the morning… any ideas?