Already left a message in another topicanswered on Super User, so just let me quote thethat answer:
When I lost my Excel password, I did quite a research only to find out that all password recovery programs can't gurantee successful restoration of lost password. If the password was long and complex, it may take years for them to find it. The only way to reliably remove password from Excel 2007 or later version (I had Excel 2010) that I have found was online service called www.password-find.com. It exploited some new algorithm that allows removing password instantaneously (removed my within several seconds). However it does not work for 100% of documents, so check it by yourself - maybe the developers improved it since then. If your document is saved in old Excel 97-2003 format you may try Free Excel password Recovery that someone mentioned already - it may work if lost password is really simple. I hope my post will save you a huge amount of time I wasted looking for some problem resolution.
So as you see, Excel 2007/2010 protection can be cracked instantaneously, and thus can't be considered strong any more.