For some reason, my Windows Mobile 5 device (a Dell Axim x51v) recently stopped sunchronizing completely with Windows XP.  I started seeing the following error whenever I'd try to sync:

"Synchronization cannot be started because you are not able to log on to the network and access information. Restart your desktop computer and synchronize again. If the problem persists, contact your network administrator."

It appeared that everything synchronized except for Notes, which would be left with the 'Attention Required' comment next to it.  On closer inspection, though, there were a number of tasks that I'd created on the device that weren't making it back to the PC's task list. 

I found a suggestion on the Internet that I should use 'scanpst.exe' to repair the Personal Folders file.  That didn't really apply to me, since my Outlook 2003 installation is synchronized with an Exchange Server, but I thought I'd try it anyway.  Once I got to the folder where 'scanpst.exe' is located, I saw that there is a 'scanost.exe' program too.  It turns out that an OST file is the local cache of the Exchange Server data. 

I ran 'scanost.exe' and let it repair all folders...and it worked! 

By the way, both scanpst.exe and scanost.exe can be found here by default:

C:\Program Files\Common Files\System\MSMAPI\1033