What version of Windows? What's giving the error message? When does it occur?
You can change the size of the D: partition using a program like Partition Magic, but it costs (I don't know any free ones). The amount you allocate to it would have to come from any other partitions you have so there has to be free space in these partitions.
What do you want to backup? There are free DVD burners you can use.
Have you thought about another hard drive?