Hi Plado Sorry I haven't replied since my first post, been busy. FAB Scott raised some very valid points.
There's only one other possibility that comes to mind, depending on whether SP2 was added as an update or not. If the new card was fitted before installing the SP2 update, it could be a driver issue, as explained here:
http://support.microsoft.com/?kbid=873169 Quite whether this would cause USB2 ports to perform like USB1.1 ports is not made clear, but it has to be worth a try.