Since update to the new version 4.6.691 the outlook starts very slow and for viewing mails it takes a long time for scanning.
Just guessing... Did you use in the past any other antivirus? Which one?
Do you have any backup (second, non-resident) virus scanner in your computer?
Can you check which plugins are available into MS Outlook:
Outlook 2003 > Help > About > Disabled items
Outlook 2000 > Tools menu > Options > Other > Advanced Options > Add-In Manager.