Before you suspect hard parts, check the inside of your belt for glazing (shiny spots.) The "inside" of the belt is, natch, the ribbed side (just in case...)
If you see any shiny bits, get a new belt. Also, get a softish wire wheel (a brass or bronze wire wheel is great for this) and use it to gently hit all the ribbed pullys on accessories - paying particular attention to your power steering pump and your alternator (since both are driven constantly, and have a greater effect on driveability.) You're not looking to remove material from the pully, just break glaze - so a gentle touch is all you really need.
Belt glazing is more common than most people think, and causes the sort of symptoms you describe. Anytime the belt itself squeals, it's losing traction on the pully surfaces - which causes glazing. Glazing causes loss of traction - as you can imagine, it's a cycle that builds on itself. Replacing the belt and cleaning the pullys will usually see to this.
You can also use a Scotch-Brite hand pad to break glaze on pullys, but it's a bit more difficult (and requires more muscle and care.) Also, you can use a steel wire wheel - but be sure to be gentle! You don't want to gouge any pullys...
