Manual solution in 6 steps:

1. Rename the file and remove the .xlsx or .xls extention etc e.g. if my file is called test.xlsx I remove the '.xlsx' - you will get a warning message saying this could make the document unstable - accept it
2. The document icon should now have turned white (the document won't look like an excel document anymore)
3. Open a blank excel document by hitting start -> all programs -> Microsoft office -> excel (this may be different if you’re not on windows 7)
4. In your blank excel document hit file -> open
5. Navigate to the document you renamed above and select it, now click open
6. It should now open the document

I hope this helps someone, if not, visit some authoritative resources...

http://www.filerepairforum.com/forum/microsoft/microsoft-aa/excel/1639-h... - any info about corrupted excel files
http://www.xlsx.openfiletool.com/ XLSX Open File Tool - paid, but fast and very effective method
http://hubpages.com/forum/topic/119472 - solutions from Excel IT professionals