Ok to recap then:

You have uninstalled norton and yahoo instant messagener
You have installed all the windows updates
You have rebooted

And now you are waiting to see if the error comes up?

After thinking about this a while, its quite likely norton is the cause of the error. See the way norton works, it scans every file you open when you open it...and if its messed up you would likely get an error..

Still no error yet?