It’s important for all users of any version of Windows to keep it updated with the latest patches from Microsoft and it should be as easy as clicking the “Check for Updates” button but that’s not always the case. Certain third party firewalls may prevent Vista from checking for updates.
In particular the error “Windows is unable to check for updates, error 80070643″ may occur. A Microsoft support document says this is related to Office 2003/Office 2007 regarding the Office Source Engine which may have been corrupted or missing. The article suggests replacing the ose.exe file from the Office CD or uninstalling and reinstalling office.