A large Windows folder is quite normal. The Windows folder may start out in the 6-8GB range bbut it will grow larger over time. This is normal behavior of Vista/7 and there isn't really much you can do about it.
The cause is winsxs, and the details are described in this blog-post.
For comparison my Windows folder is 29GB, a bit bigger than yours.
The recommendation of running Disk Cleanup in the other answer are worth a try, but I suspect winsxs is the main cause of your large Windows folder.
EDIT Filling in from the comments:
Another folder that can grow big is the Windows\Installer folder, a protected system folder that contains repair and uninstall information for the programs and updates you've installed. There used to be a program by Microsoft called miszap.exe that could clean out orphaned files from here, but there were issues and it is no longer supported.
The fact is that there really is no safe way to clean out stuff from the Windows folder beyond what Disk Cleanup can do. It is also quite normal for the Windows folder to grow over time as updates and programs get installed onto the system.
The one way to get the Windows folder back to a smaller size is to wipe the system and clean-install it from scratch. But even this is a temporary fix as the Windows folder will start to grow again over time.
To keep it from growing too much it is worth installing windows from media with the latest service pack installed and to be frugal in what programs are installed.