A/C problems?

[Guest]

I have a 99 exp, 4.6, 4x4 with roughly 90k miles. The truck is running good but when I turn on the A/C the engine starts to make a whinning sound. The engine performance is weaker but the whinning sound is annoying and sometimes embarassing. The only time I hear this is when the A/C is turned on. I can turn on just the outside fan and no annoying sounds. Any ideas?