The project that coder777 provided a link to looks rather dated (it's a Visual Studio 6.0 project!) and only deals with .xls files, not the newer .xslx format (introduced in Office 2007.) The codeproject.com page does also warn that its code only a subset of the functionality ("it contains only very basic functionality...")
It is interesting in that it works without Excel, using the Structured Storage API, but that's why it's limited to old school .xls files; .xls (and Word .doc files) are structured storage files whereas .xslx (and .docx) are structured zip files containing XML files plus resources.
One of the ways Microsoft exposes the Office functionality is via the Office COM Automation API, which does require Excel to be installed on the target machine.
C++ app automates Excel (CppAutomateExcel)
I did find an open source project: SpreadsheetLight
But they only support the newer .xlsx format. (I didn't look too hard, though.)
Office 2010 for developers
And in case you're interested, this project provides an app which can be used to open a compound document /structured storage file to see what's inside.
Another OLE Doc Viewer but with editing facility