A) make sure that when you are selecting "all files" you have included hidden files on the file system (Tools > Folder Options > View tab > check "Show hidden files and folders")

To delete all of your restore points except for the latest, go to START > RUN > type "cleanmgr.exe" without the quotes. Not only will this give you various ways to clean your hard drive, but if you go to the "More Options" tab you will see that the third category is "System Restore." Remember, clicking this button wipes all of your restore points besides the last one made... so if anything goes wrong, you will only be able to use the last one.

B) http://support.microsoft.com/?kbid=315688

C) goto computer management by click start-run-compmgmt.msc to check the Disk management or Disk Defrag and give us a screenshot for the partition info.

Since in windows it can show 31GB space for the whole partition, most likely some system hidden files are using the space e.g. MFT Zones or Restore Dir. So if A and B can not solve your problem, best way is give us the screenshot for the disk part info and maybe before that you can merge the partition of F &G to see the result or check if the system restore is enabled for drive G.