From my experience, that usually means a memory problem - a need of more memory.
How much memory is installed in your machine?
Are you running anything else when you have Outlook open? If so, quit all the other programs running, then give it a try.
If it still freezes, then you need to allocate more memory for Outlook from Outlook's Info page. Quit Outlook. Go to Applications and click once on Outlook's main icon. Now go to the file menu and select Get Info (or Command + I). At the bottom of the window that opens change the preferred memory to a higher value - adding say 1,000 KB.