I had all belts replaced abot 3 months ago (including the timimng belt), What started out as a little short screeching noise every time I turn the car (or A/C) on is now becoming an unbearable pain in the a$$ !

With the cold weather and especially with any moisture in the air , the car now has a very bad screeching noise ( I am guessing from the belts) it starts with the car and eventually dies away when the car's speed gets constand ( it now takes upto 2 minutes !!) and every time I slow down the screeching starts back up again - I also noticed a drop in MPG from around 30 to 25 !!

Are the belts too tight or too loose or is it something entirely different ? ( I assume it was the belts because it started as a little (1-2 second)noise right after I had them replaced.
Can I do this fix myself?
Thanks in advance for all help
drooby