Probability of calculating cyber attack

jhingi

Folks, need some help:

I have a tool that reported 500 virus attacks on Operating systems. I have 3 OS's Win 2000, Win NT & Win XP. Would it be possible to calculate the probability of preventing an attack if the systems had been upgraded to Windows XP? Let's say of the 500 we have 220 attacks on Win 2000, 260 on Win NT and 20 on Win XP? Other way to look at the questions is would it be possible to calculate that X% of 500 attacks could have been prevented if they were running XP? Thx!