No way of knowing for certain. You could try and see if you notice a difference with a full and empty fuel tank first! From a "fundamental laws of physics" point of view, adding weight is usually a bad thing because whatever you add is just extra weight that the tyres have to "push" round the corner. You might find that it has the opposite effect and causes the tail to swing out. However, the suspension geometry will change with the load in the car, so you could also find that it puts the tyres at a more advantageous angle. What you might find is that traction on slippy hills is worse (partly because you're taking weight off the driving wheels (assuming this 100kg is behind the rear axle) and partly because you're making the car 100kg heavier to lug up the hill!
My gut feeling is that if the money's there, you're probably best off forking out for a decent set of winter tyres, but I know that's expensive and this isn't the time of year you want to be shelling-out for stuff!