How to recover forgotten password in MS-Excel?

Hi All,
My sister asked me to open a Microsoft Excel document in our desktop which contains her passwords and accounts. She forgot the password for one of her accounts that she need to logon to. The file is password protected, and I just know that it should for security purposes.
Now, my problem is, the password that she gave me is not working. I already tried twice and I am not sure what will happen if I got the third incorrect try. Anyway, my sister is waiting for my reply and I told her already about the password not working. She confidently told me that the password she gave me is correct. I checked and all our keys in the keyboard are all working. I cannot think of anymore option, is there any software that I can use to recover the password? Please help me.
