I can't confirm if you have a big windows.old file with backed up data created.
When you use windirstat tool to check disk usage, does it not show large file occupancy?
I consider that you could use system tool (disk clean up tool) to check firstly.
Start>all programs>accessories>system tools>disk cleanup
If you want to recover more disk space, you can go into system restore and there is a slider there that will allow you to adjust the amount of disk space you want to allow system restore to use.
At last, you may try to use Treesize tool to check what's taking up the most amount of space.
Hope this helps and please help to accept as Answer if the response is useful.