There are two alternatives here:

1. There is a problem with the work unit. If this is the case, then all the copies of the work unit will fail, and the work unit is automatically reported to the WCG techs for further analysis of the problem.

2. There is a problem with your computer. This is far more common. Overclocked computers frequently return invalid results. We advise members not to overclock. Other sources of errors are memory, failing hard drives, and sometimes software conflicts such as virus scanners interfering with the result.

If you have invalid results very rarely, then don't worry about it. If you are seeing frequent invalid results, then you need to look into it further.

Visit the grid.org forum, no sorry that was eradicated, on the number of times a virus definition update caused for this to happen. The notorious are mentioned honourably in a Start Here forum FAQ.... sometimes the WCG techs get involved (probably banging the AV maker's door while wearing a white IBM shirt and tie)

I don't think it would be Norton. Unless there is some setting of which I am unaware. I have that one system which my son's University account requires(downloaded from the there) and have Avast on others. Have had no problem with either one.

Cheers