+1 for using Disk Cleanup.
The Windows OS partition size increases over time, with use.
For example:
- Windows Updates
- System Restore points
- Installing software
- Updating software
- Backup installers
- Windows Logs / Events etc.
- IE and other Browser Temp files, cache, history etc.
- Windows Features (WMC, Games, IE etc.)
- Also you enquired about the Windows.old folder, that takes up space too.
You can always use the Quota, so your partition drive doesn't reach it's max. capacity.