Taking ownership of the complete C: drive (assuming this is the system drive), is not a good idea as many of win7 ' s security functions relay on ownership being held by system or Trustedinstaller.
If you have to take ownership you could try going into safe mode as the hidden administrator and trying and setting the owner there.
If you do this please be aware that the resultant system may become unstable or unpredictable
If you need permissions on certain folders for a special purpose this can also be achieved on a per folder/file basis by use of the hidden administrator, and the potential for mishaps will be reduced.
I would also recommend that you add permissions rather than take ownership