Back up from Pavilion HDD used externally

Hello,

I have an HP desktop with Windows Vista where the Windows is faulty and will not Boot. I had not backed up the Data prior to this. I put the Hard Drive into an External Enclosure hoping to be able to back up the files to another PC running Windows 7 before running the recovery discs, however it will not allow me to access the folders where the Data is stored.
How can I access this Data?

Thank you and kind regards,