Problems with low disk space on C: partition... HELP!?

I am having trouble after a re install of windows 7. I installed it onto a 15gb C: boot partition and I have a 250gb hard drive, but I am receiving a "low disk space" error message and am having run errors... performance seems to be lacking. I tried to extend the boot volume but it will not allow me to. What did I do wrong in the install and how can I fix it?