Yes, I know the solution to this problem may be easy to find (just Google the exact error), but as a developer I try and optimise most things that I do (well, except my circum-verbose writing style), so I am just going to “re-index” it here to make it easier to find (for me).
If you are using OLEDB to read .XLS files into your application then in the end you are using the 'Microsoft.Jet.OLEDB.4.0' provider. The problem comes in when you try and develop on a x64 system.
At this point you will end up with this error:
The 'Microsoft.Jet.OLEDB.4.0' provider is not registered on the local machine.
So... The solution
Simply change your project to build as a 32 bit (x86) application and all will work fine.
Should this not solve your problem... do some reading :)