This could be so many things:
1: as above poorly fitted pads. make sure the pads are able to be slotted in to place easily but do not have too much free movement. Apply a strong layer of copper grease to any areas that contact the calliper. DO NOT GREASE THE FRICTION SURFACE
2: Pads being worn. Some, but not all, pads have a squealer arm which will contact with the disc to let you know the pads need changed.
3: discs are warped. This means that the disc is infact oval and contacts with the pad at certain points in the rotation. Causing a squeal.
4: Dirt, grit, mess in and around the calliper/pad/disc. Double check that all is clean as can be and in working order. Wire brush carriers and callipers if need be.
5: The pads themselves. Some pads have a much higher metal/carbon ratio than others. Usually found in cheaper pads it makes them last longer. Can also make them squeal.
I would like to add a caveat on the last point. Some performance pads such as the Ferodo DS2500 will also squeal. This is because their primary concern and in fact design is geared toward stopping the car under harsh braking this means that there is little consideration to the sound produced. My own ferodos squealed when cold, when hot, when stopping. Just about all the time.
Hope that this helps
PS Anti Squeal shims. Get these to hell. You do not need them. I have never, ever used them and see no need.
Neil