Problem Loading Exel 2010 files in Exel 2013 (Office 365)

I have several computers and previously used exel 2010 and have written several large Spreadsheets that operate comfortably in that environment. I recently purchased a tablet computer running Windows 8 and subscribed to Office 365 which includes exel 2013. Whenever I try to load a file created in the exel 2010 environment I get the message Error cannot open file "Filename" because the file formator file extensionis not valid. Verify that the file has not been corruptedand that the file extension matches the format of this file I have followed several advices from different help areas, have unchecked all the option in the "Protected View" options but still this problem persists Can anyone please shed some light on this? Thanks

Thanks for the offer of help Dave, I have found the solution to that problem only to be now confronted with "reference" issues in the VBA environment. Should have stuck with 2010, can't say I wasnt warned tho