Which Of These Ads Is Delivering Malware?

People are getting reports on one of my blogs that malware/viruses are being downloaded.
I have the following ads running:
Adsense
Adbrite
Adtoll
Clicksor
Kontera
Neoffic
Smowtion
XaPAds
Has anyone found these to be running malware lately?
I run the exact same ads on two other blogs, why is malware only being shown on one of my blogs and not all three?

See original post by Dean_Saliba