The following is an excerpt from page 14 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding Combat Resolution and from wounds caused by Killing Blow.

Several people have asked if a successful Killing Blow can be regenerated, and also how many wounds does it count as for Combat Resolution? Well, taking the first question, a Killing Blow or any other attack (such as from a magic weapon) that kills instantly rather than removing individual Wounds, cannot be regenerated. It's just that - a Killing Blow! Secondly, the number of wounds it counts as inflicting is equal to the number of Wounds the dead model had before it was killed. For example, a model with 3 Wounds remaining that is felled by a Killing Blow would add +3 to the Combat Resolution score. The same applies to other 'instant death' attacks. This works slightly differently in challenges, but I'll be talking about those elsewhere.