Verified answer

90%. If it didn't catch a virus on the first try, it'll miss it on the second as well.

If it took out 90% the first time, and 90% of the remaining viruses the second time, then it would take out 90% + (90% of 10%) = 99%.

Another way to figure that is that 10% survive the first use, and 10% of those survive the 2nd, which would be 1%, again giving 99% effectiveness.

That 2nd approach might make sense for some applications, but not for cleaning computer viruses.