To do this edit this DWORD registry key: (Only affects NTFS partitions)
Note the utility fsutil can also be used to change this value:
[HKEY_LOCAL_MACHINE \System \CurrentControlSet \Control \FileSystem]
Note that original Windows 95 without SP1 will not be able to access files created.
fsutil behavior set disable8dot3 1
Note that some applications still uses 8.3 filenames, and might fail to function (Ex. applications running at boot time ex. virus scanners or similar).
More Info MS KB121007
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