Hi - unless I am out of date - my understanding is that the SyncToy tool will synch entire data files (e,g. your Outlook data file) and not individual items (e.g. contacts, calender items, etc.).
If this is still the case then your Outlook data file needs to be not in use one either PC prior to running SyncToy to synch the data files to have the most recent version overwrite the earlier version, in it's entirety. If you are using a utility to access Outlook to synch with another device (e.g. your PDA) then this utility needs to be shut down to ensure that your Outlook file is not in use.
Whenever I receive this message, it is that I forgot to shut down my PDA synch utility and it is still access the Outlook data file.
I hope this helps.