Try first to launch Outlook in safe mode (Outlook /safe).
If the problem disappear in safe mode, try first disabling add ins and
then try renaming cache files (source : newsgroups)
Locate and rename the Outlook cache files
(frmcache.dat,outcmd.dat,extend.dat) one by one.
1. Quit Outlook.
2. Click Start -> Search -> For Files and folders
3. Click Tools -> Folder Options.
4. Select View tab, select Show hidden files and folders, uncheck Hide
extensions for known file types, and click OK.
5. Click All files and folders.
6. Type "frmcache.dat,outcmd.dat,extend.dat,views.dat" (without the
quotation marks) in the filename box, and then select Local Hard Drives.
7. Click More advanced options, check Search hidden files and
8. After the search has finished, renames files in .old
NOTE: We may find multiple files for each name. Please rename them all.
9. Restart Outlook.