My alternator was recently changed and now what happens is there is a horrible little screech (for a split second) when you first start the car, but only as you start the car and mostly if it sits overnight.

I'm thinking the belts are not tight enough am I correct?

 

Belts are probably too loose. They stretch a little at first which requires a little adjustment to make them thighter. Take the car back to where you got it done and have them adjust them.