AC Problems

So after the drama of our plumbing mess, we thought all of our problems with our home was over with. Not so, now our AC is freezing up and not cooling the house. Our house is really, really old, so we have window units. Well this unit is pretty much new, around 5 yrs old. We have another unit that came with the house and is probably 100 yrs old and runs perfectly. I guess they don't make AC's like they used to. Anyway we need to figure out if this is a problem that can be fixed or if we just need to purchase another unit. Looks like we will be having to dish out some more money.