Error: 1301

Everytime I try to run an update for Adobe Acrobat (in this case to version 9.4.2), I am met with this error message:

The update could not be applied because the Adobe Acrobat application contents were modified since the original installation.

Please reinstall Adobe Acrobat and check for updates again.

Error: 1301

I don't want to have to reinstall Acrobat everytime there is an update. Can someone tell me how I can fix this?

Thanks!