I recieve the follow error message

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Hi when try to erase unused disk space on both my C, D and external hard drive I recieve the follow error:
1 Error: Erasing the Free Space of a drive requires elevation
2 Error: Failed to erase file names from drive H:\ (C or), dont know why exactly.

Im using version 5.8.7 on windows vista.

Any help in this matter would be greatly appreciated.

Thanks