We'd suggest SpaceSniffer or MeinPlatz because most other options are neither portable nor entirely free.
We want to use a portable app as you'll probably only need the app one time. Your best bet is to use a third-party app. A previous Windows installation comes to mind (windows.old). There are many situations where you could have a large file or folder using up space that you're unaware of. TIP: Hard to fix Windows Problems? Repair/Restore Missing Windows OS Files Damaged by Malware with a few clicks