What does Disk Management say about the disk - is it showing as the correct size there, or extended beyond it's "max space"?
Try running DIsk Cleanup and when it's finished it's first scan, click the "Scan system files" option as well, and re-scan.
I'd also try dropping down your System Restore allocation percentage (System -> Advanced System Settings -> System Protection tab)
Also under Advanced System Settings, under the advanced Tab->Performance Settings, check what your Page FIle / Virtual Memory is set to.