Windows

When you do Ctrl-Alt-Del what task is not responding?
If you are getting an MPREXE (not responding) rebuild the password list file as described below:
1-On the Windows taskbar, click Start, point to Find, and then click Files Or Folders.
2-Inthe Named box, type the "*.pwl" (without quotation marks) and click Find Now.
3-Right-click the file titled <username>.pwl, and click Rename on the shortcut menu.
4-Rename the file to <username>.old, and then press ENTER.
5-Reboot and enter your passwords.
If this is not solve the problem, remove Outlook and reinstall it and set up your services from scratch.
Good luck!

If you are using Word as your e-mail editor, try changing the setting to Outlook Rich Text. There are "known issues" (ie., bugs) with Word and Outlook - this sounds like one of them.

The user profiles could be damaged.
If it is the same user who are repeatedly having problems, you could try deleting the *.pst files one each users system. Outlook will re-create the files the next time it is opened.

Good Luck.

All answers are good and could this type of problem but something you should consider is do you have a Anti-Virus program set up to automatically scan your incoming e-mail?
If you do this could be causing a problem that is causing the freeze, I hadthis exact thing happen to me at home and fixed it by reinstalling the antivirus product.

This should definitely cure your problem. Go to the link (or copy and paste) and download the solution. http://www.microsoft.com/technet/security/bulletin/MS00-017.asp