What's the difference between an abstract class, and a class with only protected constructors? (.NET)

What are all the difference between an abstract class, and a class with only protected constructor(s)? They seem to be pretty similar to me, in that you can't instantiate either one.

EDIT:

How would you create an instance in a derived class, with a base class with a protected constructor? For instance:

public class ProtectedConstructor
{
    protected ProtectedConstructor()
    {

    }

    public static ProtectedConstructor GetInstance()
    {
        return new ProtectedConstructor(); // this is fine
    }
}

public class DerivedClass : ProtectedConstructor
{

    public void createInstance()
    {
        ProtectedConstructor p = new ProtectedConstructor(); // doesn't compile
    }

    public static ProtectedConstructor getInstance()
    {
        return new ProtectedConstructor(); // doesn't compile

    }
}