Solution 1: Close Outlook. Rename or delete outcmd.dat, extend.dat,
views.dat, the .srs file, and frmcache.dat. Outlook will recreate these files
as needed.

Solution 2: Create a new PST if you use one. Although the Outlook 2007 PST
is essentially identical to the PST format new to Outlook 2003, Outlook 2007
writes additional data to the PST.

Solution 3: Create a new profile. No need to delete the old profile just
yet, just make a new one to see if this resolves anything. If you upgraded
from Outlook 2003, then it's highly recommended you create a new profile.

Solution 4: Disable Outlook Setup Add-in (Cyberlink). This is the Instant
Office add-in on Dell computers that have Media Direct installed. This
add-in is responsible for converting meeting requests to recurring meetings.

There are some other potential issues that may slow Outlook down as well:

Instant messenger. The fix is to disable the link to Instant Messenger on
Outlook's Tools, Options, Other tab.

Network connectivity issues when you are using mapped network drives,
including slow networks or inaccessible mapped drives. The fix is to uncheck
the box for the dialog to always ask whether to open/save the attachments or
disable mapped drives.

Alternative input support in Office 2003. This allows you to use speech or
handwriting instead of typing. Disable alternative input following
instructions in How to turn off the speech recognition and the handwriting
recognition features in Office 2003:

http://support.microsoft.com/?kbid=823586  

Other options are to archive, run disk checking tools like chkdsk, scandisk
and defrag. There is also the option of running detect and repair from the
help menu. If a PST is in use, run the inbox repair tool. If there is a
desktop search program installed, get rid of it.