I had exactly the same problem when i picked my car up.
the garage had put new back tyres, kaiser something on the back to get it through its mot before i picked it up.
I nearly spun on the wet motorway on the way back, slightly damp roads or any kind of change in surface / camber, markings on the road would really unsettle the car. it would crab about all the time and even with the TC off the slip light was coming on all the time and the brakes were grabbing at random wheels. it was very scary.
I actually posted in a thread called 'suspension problems' on here.
anyway. it was the tyres. I couldn't believe the difference it made and now i would never put even a mediocre tyre on my car.
if you have a fairly powerful rear drive car you need good tyres!!!
I would get them changed fast, and make sure you put the same on the back as the front, or get new tyres all round
Lets not turn this thread into a battle about which tyres he should get though, we have enough of those allready.