Squealing brakes

i have a kia sportage 2000 and the brakes keep squealing, what could be the problem?

How long has the squealing been going on?
How many miles are on the odometer?
When was the last time that the front brake pads were replaced?
When was the last time that the rear brake pads were replaced?
Does it squeal when you apply the brakes?
Does it squeal when the brakes are not applied?

they have been squealing for about a week, there is 80.000 miles on the odometer, front pads were replaced about 2 weeks ago and rear brake pads were replaced about 2 months ago, it squeals when the brakes are applied and when they are not applied, but mostly when they are not applied.

You need to take the car back to whoever did the most recent brake job. It is not unusual for some disk brakes to make noise, particularly right after they have been replaced. They may simply need some anti-noise compound applied to the back of the pads, or it could be that there are other problems, but you should bring this problem to the folks who replaced those front pads 2 weeks ago.