Can't change (enable/disable) features on Windows 7

I can't make any changes to Windows Features. Even if I just enter in Windows Features and press Ok I get: "An error has occured. Not all of the features were successfully changed."

I noticed this while trying to install/uninstall IIS. If I try to install IIS from the Web Platform I get:

[Windows package manager]
The referenced assembly could not be found.
Operation failed with 0x80073701

Any suggestions?

EDIT: I have tried
-> Startup Repair. It says it has found problems but is unable to solve them
-> System restore (but didn't had an early enough restore point)
:(

0x80073701 is "ERROR_SXS_ASSEMBLY_MISSING".

Sounds like WinSxS (%windir%\winsxs) folder corruption, and/or missing files/links within.

Have you done a disk check (chkdsk)? And/or and a System File Check (sfc /scannow).

Check the windows Event logs for related events to ensure you are not having something like disk or disk controller errors.

Make backups before repairing any disk errors. :)

Also, the System Update Readiness Tool for Windows 7 (KB947821) can help diagnose problems related to a corrupted WinSxS.

You may also have to/want to re-apply your last service pack after the disks have been declared clean and correct.

Additional info here, and here.