the program is very good, i used it recently. it says your system drive is only using about 17gb in total ! i'm not sure that is possible, no wonder you have problems though, if the system is complaining that you are running out of space, then the drive (partition) needs to be bigger. you are supposed to have a decent portion of breathing room for the system, and then space for you to do your tasks, and add files etc.
for now you can disable system restore, do disk clean (including the actual system button there) and disable hibernate to gain some space.
disable system restore:
1. right-click start menu button, click 'system', to the left click 'advanced system settings'.
2. go to the 'system protection' tab, click the 'C' drive in the box, click 'configure' button.
3. at the top select 'disable system protection', click 'ok' button at bottom.
disk clean - including system stuff:
1. in the file explorer, select and right-click the 'C' drive, go to 'properties'.
2. click 'disk clean-up' button, select the 'C' drive if it asks, click 'clean up system files' button.
3. scroll through the list and notice if any size ends with 'GB', highlight those and see what the information says about if they are safe to delete - they probably are. if you are concerned, post back here with the information.
disable hibernate:
1. type 'cmd' into the start menu search, and then right-click 'command prompt' to choose 'run as administrator'.
2. copy and paste the following command
into the command prompt window and press Enter key.
now restart the computer.
as for how to use the nice WinDirStat tool, what you do is simply view. focus on the top-left side of the stats. the collumn headers 'name', 'percent' and 'size', also 'subtree percentage' for quick visual guidance. 'size' tells you the total size of that folder/file, the 'percentage' tells you how much of the total 'C' drive size is being taken up by the relevant item. click on the plus signs to the left of each main folder to dig deeper. for now though, do the above mentioned things and see what state it is afterwards.