Below is a small guide on how to remove Microsoft Excel passwords from a "locked" spreadsheet/workbook.

Required Software:
WinRAR/Winzip/7zip (I have only tested with WinRAR but I suppose it would work on others
Notepad

Has been tested on:
Office 2010 (.xlsx)

Step 1
Backup whatever file you are going to be tinkering with. Its extremely good practice to backup any file that you decide to make unorthodox changes too, and this is no exception.

Step 2
Double check that Excel is closed, and then [Right Click] the file you intend to remove passwords from, select {Open with} --> {Choose Default program...}

Step 3
Remove the check from "Always use the selected program to open this kind of file"
Click {Browse...}
Locate the folder containing the WinRAR executable, highlight it, and select {open}
Click {OK}

Step 4
Navigate the "zip" to ----> \xl\worksheets\
Open the worksheet that you wish to remove protection from (sheet1.xml, sheet2.xml, etc...)
(most computers will open the file up with a browser)

Step 5
Look for lines of code that would indicate "protection"

An example would start with
Step 6
Open the file with notepad and completely remove the line of code. Save it and close.

Step 7
Open the excel file normally and enjoy your password free document!

If you are trying to remove workbook restrictions, the file you need to edit is \xl\workbook.xml