Hi Paul,
the CRM must be upgraded at least to rollup10 and the outlook client for CRM 4.0 will works only with 32-bit of Outlook 2010 (Windows can be 32 or 64bit).
After the upgrade the users must install the last version of the outlook client, you can find here:
http://www.microsoft.com/en-us/download/details.aspx?id=1079