First, you're most likely specifying an absolute directory ( "C:\Blah\BlahBlahblah..." ) or you've set the Working Directory to the wrong place.
Use relative paths or always store your data in a fixed place (the convention these days is to store your data is APPDATA. Storing it in a place like Program Files is a HORRIBLE CONVENTION. Don't do it when you release your program for realsies). Having tagged this with XNA, your root directory should also be specified to a relative place ("Content", "Data", etc.) so that when the executable boots up, it'll look in where the executable is, + inside the folder "Content" or "Data". That will make using the Content Loader easier.
Now, a general recommendation. You should never have to create a new project entirely to do version control. You're literally doing the work of any good free Version Control System (or like this one). Version control will make this much less painful for you.
Finally, a last tip for SO: describing your problem is cool, but what's even better than all of that is combined is posting the code that throws an error. Doing this allows us to figure out what exactly what wrong, because the skilled professionals of SO (of which I am trying to become one) have dealt with and know how to handle many kinds of exceptions and compilation errors (that, or our Google-Fu is stronk).