Hi,
Have you had a look at the following link from the National Instruments website? It might be worth going through the following steps in the Knowledge Base document at the link as the first thing to try. If the problem is still there, post back and I will see if I can find some further information as to why this is happening.
http://digital.ni.com/public.nsf/websearch/53149DDE0296918286256FCB005FCE6F?OpenDocument
Hope this helps.
Regards
Lilly N
National Instruments UK