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XP: C:\Documents and Settings\Local Settings\Application Data\Research In Motion\BlackBerry\Intellisync (I think, it's been a while since I've used XP)
Vista: C:\Users\"Insert your user name for your computer here"\AppData\Roaming\Research In Motion\BlackBerry\Intellisync
Once there, open up the tif.log. At the bottom of the file, there are error messages. Mine said that something was too long. I searched from the top of the file for what it said was too long. From there I was able to figure out what it was that was causing me problems: An event that I recently added to my Outlook Calendar. Once I deleted that event (luckily I didn't need to have it in my calendar) my problems went away.