Problem is with "Hidden" file location on HDD.
Try disabiling hibernation, reboot and delete hybernation file. File is hidden file and normally stored toward the end of the HD and you can only shrink upto file location.
Might be able to use disk cleanup to delete file. under accessories, system tools .
a 2nd file problem may be the "system rstore file' See my link at end.
I had this problem about a year ago on wife's "Store bought - ugh" system and that is what I found after doing a google search. Was able to shrink partition after following suggestions in link. Would have elaberated more, but have to go bowling.http://www.howtogeek.com/howto/windows-vista/working-around-windows-vistas-shrink-volume-inadequacy-problems/