1) Compact the database:
Open the database in Access, go to "Tools" -> "Database utilities" -> "Compact and repair database.."

2) Compact your Outlook ® datafile:
http://support.microsoft.com/default.aspx?scid=kb;en-us;289987

3) Search for the following files, and rename them:
"outcmd.dat"         --->       "outcmd.old" 
"extend.dat"          --->       "extend.old" 
"frmcache.dat"     --->        "frmcache.old" 

4)  In Outlook ®, go to "Tools" --> "Options" --> "Other" and remove the "Enable Instant Messaging" check mark.

5) Exchange Server Users:
http://support.microsoft.com/default.aspx?scid=kb;en-us;328880&sd=tech