Used space/free space on HDD erroneous readings

Running vista home premium 64-bit with 600GB HDD. When I open "Computer" it shows graphically and numerically the used and free space on my drives. C - normally shows 310 GB free of 585 GB, D - Factory image - 1.47 GB free of 11.1 GB, K - HP Pocket media drive 237 GB free of 465 GB. The problem I am having applies only to C, with the others remaining unchanged. The graphical representation of free/used space is in blue and white normally. Here is the problem. The last couple of weeks when I open"Computer", I sometimes get C - 50.6 GB free of 585 GB and the graphical representation is in red, not blue. I got an error message the other day that I did not have free space to do what I wanted to do. When I checked free space it was showing, in red, free space of 50 GB instead of the normal 300 plus GB. As soon as I reboot the free space is shown as the correct amount - 300 plus GB. Anyone know what is going on and how I correct it?
 
I would cleanup like this

on your normal login
  • get a cmd prompt and enter cleanmgr

do it again using an admin login

Then verify the free vs total space
 
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