FOR OTHERS, here are things to look at:
1) What tool(s) are you using to examine the disk?
Particularly with system drives, I often leave a large part of the drive not partitioned. For instance, with a 128GB SSD I might partition 70GB and leave the rest of the drive unallocated. That should be more than enough for programs, etc., and not wind up with 50+GB of Windows' extra "junk" files that would just increase the size of backups. If in the future I need more O/S space, I can just resize the partition. And if at some point I need an extra high-speed partition--hey, there's a bunch of available space. For instance, awhile back I partitioned some unallocated space and moved all my genealogy stuff to that.
At work, we buy refurb PC's that come with 160GB HDD's and I have to change the O/S to 32-bit. Realistically, the users need less than 30GB, since the data is on a server. I'll normally create one 60GB partition, so if in the future I need to clone the drive to an SSD I don't have to worry about the drive having more than 128GB of files.
A lot of tools won't show unallocated space.
2) If you really need more space, move or delete the SoftwareDistribution folder.
When Windows does updates, it stores the installer in C:\Windows\SoftwareDistribution\
and keeps it forever. Over time that can get really big. Do all updates, make sure everything runs okay, then in Services turn off the Windows Update Service, then rename the folder, then delete the renamed folder or move it to another drive, then in Services turn Windows Update Service back on. (You have to rename the folder or Windows won't let you fool with it.)
When you do that and then run Check for Updates, it will show Last Update as Never. It doesn't roll back your system, it's just that Windows assumes that since there's nothing in the folder the reason is because you never updated. Windows will automatically create a new empty SoftwareDistribution\ folder when you turn Windows Update Service back on.
One caution on this is that it's [probably] not reversible--because Windows doesn't know you updated something, if an update does cause a problem, you can't remove the update. You [probably] can't just turn off Update Service again, put the old file back, turn Update Service back on, and be back like it was before you started.