What can I safely delete from the "C:\WINDOWS" folder?

The biggest stuff in C:\Windows should be the winsxs folder, read more about that problem Why does the /winsxs folder grow so large, and can it be made smaller?. Since that stuff is used by all kind of apps, you must not remove it. And since it is the biggest chunk (of "junk"), all other directories do not matter that much any more.

You can obviously remove some leftovers / rollback-stuff, best done via http://www.piriform.com/CCLEANER.

To make it short: Buy a bigger disk.

I've written a Blog post on the SuperUser Blog detailing some of the things you can do to scrape back some hard drive space. Read it at Where did all my hard drive space go?

To summarise, there are many things you can do, depending on whether there is functionality that you need:

• Don't use hibernation? Then you can disable it and claim back a quantity of hard drive space equal to the amount of RAM you have, you just need to log on as an administrator, open a command prompt and type the following two lines:

  powercfg –h off
del C:\hiberfil.sys

• Old Windows Update files can be deleted as the folder they are stored in used to get quite big. They are stored in C:\Windows\SoftwareDistribution but you'll need to go through the method I detailed in the blog to properly clean the directory out.

• System Restore is another hog and deleting old restore points can clean out space, you can even control how much space is available for use with System Restore

  As an administrator, simply click the Windows Start menu icon, enter “SystemPropertiesProtection.exe”

• The WinSxS folder is a red herring and contains no data that is not already duplicated elsewhere and deleting it will save you nothing. This special folder contains what is known as a hard links to files which are scattered across your system and are kept in that folder to simplify matters slightly. The hard links take nearly no extra space in the filesystem as they are simply another pointer to already existing file data.

An alternative to the excellent CCleaner that Akira linked is a more commercially supported alternative from IOBits, Advanced SystemCare4. There is a free version available which will likely do just as much to clean out your system as CCleaner, but has a few added extras.

Since October 2013 the disk cleanup utility was enhanced with the ability to clean up Service Pack and Windows Update files, this is the safest way to reduce the sized of WinSXS folder.

KB2852386 is what added this feature and most W7 PC's have it installed now.

Cleanup first pass

Click Clean up system files button and this is what you will see.