I'm using Windows 7 64bit Premium for two years now and in this time my C:\Windows folder has grown to almost 30GB which is, in my opinion, inacceptable.
The biggest subfolders are winsxs (10.5 GB), Installer (5.5 GB), System32 (4GB) and SoftwareDistribution(2.5GB). I googled for the winsxs folder and found that it's important and may not be deleted. I googled for the Installer folder and found that it's mostly Office-related and that it might be reduced a bit by deleting Office updates.
But that's not satisfying. Does it really take 27GB to have a working Windows 7 plus up-to-date Microsoft Office?
Is there any way to safely shrink the Windows folder (without having to reinstall Windows 7)?