Your problem is primarily a cosmetic problem created (but real enough when
it comes to system restore) by having so much free space on your hard
disk.  I suspect the problem will disappear once the free space drops
below 64GB or so.  The problem is due to the way large integers are
handled by the system interface.  Until free space is reduced you can
expect to see similar oddities in the Virtual Memory settings (see below)
and also possibly when running scandisk and defrag.

For a related problem and explanation see KB MS KB272620 - "Negative Hard
Disk Free Size Reported on Virtual Memory Tab in System Properties"
(http://support.microsoft.com?kbid=272620).  If you decide you want this
hotfix when you contact Microsoft give them details of the problem you are
having, your operating system and the hotfix you require.  Provided you
ask no other technical questions there should be no charge other than the
cost of your phone call.  If you have any problems ask to speak to a
supervisor and also please e-mail me details.

IMO the best way of dealing with the problem is to partition the disk and
keep the system partition small at around 4GB or so, allocating the
remaining space to one or more data drives.