Hi, here are the answers to your questions:
>> 1. Can I run LabVIEW 5.1 application on Windows XP ?
Yes.
>> 2. Is NI-DAQ 6.6 compatible with Windows XP ?
Probably not. NI-DAQ 6.6 was released before Windows XP.
>> 3. Can I use NI-DAQ 7.4 with LabVIEW 5.1 ?
No. You'll want to install NI-DAQ 6.9.3, which supports XP and LabVIEW 5.1.
>> 4. Can I use a PCI-6070E with NI-DAQ 6.6 and can I use a PCI-6070E to replace a PCI-MIO-16-E-1 without changing >> the LabVIEW code ?
The PCI-6070E is really the same thing as the PCI-MIO-16E-1, so replacing the latter with the former is not an issue.