To reinitialise the Offline Files cache, create a DWORD registry value 'FormatDatabase' with a value of 1 and restart the system. WinVista/7: HKLM\System\CurrentControlSet\Services\CSC\Parameters

Win XP: HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\NetCache

With thanks to John R. Pattison: http://johnrpattison.com/2007/11/26/how-to-reset-offline-files-in-windows-vista/