Amazing nobody has answered this in the past ~3 months. The solution is a very simple: adrot.TargetFrame="target='_blank'" added to the rotator call.
<%
set adrot=Server.CreateObject("MSWC.AdRotator")
adrot.TargetFrame="target='_blank'"
Response.Write(adrot.GetAdvertisement("ads.txt"))
%>
Good Luck!!