M = cL
M = The degree of mission criticality of an enterprise system Varying from 0 to 1.
L = The degree to which that application is considered "legacy" by the IT industry. Varying from 0 to 1.
and c is some naff constant without which, no formula can be considered complete.
Anyway, there are a number of things that you can do to mitigate the risks created as M and L creep stealthily in lock-step from 0 to 1. Virtual Operating Systems is one of them.