I have experienced the same exact symptom.
I am running Windows Vista Business 64-bit
Office Enterprise 2007
I found the answer to this question at the following link:
In my case it turned out to be a problem with the Symantec AntiVirus Outlook Protection add-in.
The explanation was not entirely clear so I have revised it and I am reposting it here.
1. Go to Tools/Trust Center/Add ons
2. At the bottom at the heading “Manage” select either Com Add-Ins or Exchange Client Extensions. (In my case the later applied)
3. Click the Go button.
4. Deselect all items and retest.
5. If the problem is corrected add each back until the problem comes back in order to identify the culprit.
I hope this is helpful.