Hello Valen Li,
Thank you for contacting National Instruments.
Try reinstalling LabVIEW. The problems you are experiencing may be associated with LabVIEW and not with the VIs themselves. Also, before compiling, try opening your LabVIEW 5.1.1 VIs in LabVIEW 6.0.2 and resaving them. The following link will take you to an article about mass compiling and exactly what it does. It may help shed some light on the cause of your problem.
http://digital.ni.com/public.nsf/websearch/654877E62A97B72986256C95006F9B24?OpenDocument
If you have further questions or problems, let me know.
Matthew C
Applications Engineer
National Instruments