1999 Chevy Cavalier When I turn on my A/C my check engine l

A year back my check engine light went on, the engine would run rough when I used the a/c, and when I let my sister borrow it and it was left on my car wouldn't start until I turned it off. Like a good little car owner I ran my vehical to the mechanic. I told him my car acted up when I had the a/c on, he said it was a coincedence. Once he got it into the shop he fixed the major problem and told me that in fact something in my a/c system was failing. Unfortunalty it cost over $700 dollars to fix and I forgot exactly what he said it was. I was wondering what it could possibly be and if I could fix, at least, most of it myself?