For Excel 2007/2010 (.xlsm) files do following steps:

1. Create a new .xlsm file.
2. In the VBA part, set a simple password (for instance 'macro').
3. Save the file and exit.
4. Change file extention to '.zip', open it by any archiver program.
5. Find the file: 'vbaProject.bin' (in 'xl' folder).
6. Extract it from archive.
7. Open the file you just extracted with a hex editor (for example HxD).
8. Find and copy the value from parameter DPB (value in quotation mark), example:
        DPB="282A84CBA1CBA1345FCCB154E20721DE77F7D2378D0EAC90427A22021A46E9CE6F17188A".  (This value generated for 'macro' password. You can use this DPB value to skip steps 1-8)

9. Do steps 4-7 for file with unknown password (file you want to unlock).
10. Change DBP value in this file on value that you have copied in step 8.
>>If copied value is shorter than in encrypted file you should populate missing characters with 0 (zero). If value is longer - that is not a problem (paste as it is).

11. Save the 'vbaProject.bin' file and exit from hex editor.
12. Replace existing 'vbaProject.bin' file with modified one.
13. Change extention from '.zip' back to '.xlsm'
14. Now, open the excel file you need to see the VBA code in. The password for the VBA code
will simply be macro (as in the example I'm showing here).

For those who still using old file format (2003 - .xls) method has less steps:

1. Open .xls file with hex editor (for example HxD).
2. Find parameter DPB, change value for this parameter by our own. If your value is shorter than value in encrypted file, you should populate missing characters with 0 (zero). If value is longer - that is not a problem (paste as it is).

DPB="282A84CBA1CBA1345FCCB154E20721DE77F7D2378D0EAC90427A22021A46E9CE6F17188A".  (This value generated for 'macro' password).
3. Save changes.