Shut down the client. Delete everything except for the needed .exe and .cfg file(Uh I had to push my glasses up while typing this and rubbed my finger all over them instead). The only thing that should be left is Folding@home-Win32-x86.exe and client.cfg. Then it will download everything new.
Hopefully it is just a damaged file that the client is failing to delete in its clean-up run thus the IO_Error due to a check sum. This would make the most sense.
Is this application being running as admin and are is your user account that you are using an admin?