Taken from: http://www.straxx.com/excel/password.html

Have you ever forgotten your Excel workbook or sheet password?
Here you can download a FREE Excel add-in that removes/cracks sheet and workbook password protection in Excel.

NOTE! This program can’t remove file protection, i.e. if you are unable to open the file, this program won’t help you.
You could try Excel Key to remove file protection. If you need to crack the VBA-password try this one.

This Excel add-in (password.xla) works fine in Excel 5.0 and above (including Excel 2000, XP and 2003).
This program will remove passwords of any length, also passwords containing special characters.

Download password.xla (35 KB)

hint: after download from the above link, please change extension .doc into extension .xla

First of all, we need to set the macro security into Medium or Low (not recommended).
Open the Excel application, choose Tools –> Macro –> Security.
Open the protected Excel file then open password.xla.

Now you can unprotect the protected Excel file by choosing Tools –> Unprotect sheet/workbook.
Enjoy it!