Installed winter tires on my daughters 2002 Mustang today but when she tried driving on the highway she could only go 70 KM/hr. Wheels are not slipping but traction control light comes on. It doesn't matter how hard the throttle is pressed will not go any faster.
Any ideas, I have not tried it out yet but will try in the morning with the tracition control turned off.

 

I think you have discovered a good thing here. Next to being in the car and telling her to slow down the traction control is doing it for you.
Did you install the same size tires you removed?