Hi Jillian,
As far as I'm aware there isn't a way to repair aluminium windows that have oxidised. You could have them painted but this is quite an expensive option (you could spend half your budget just on the windows).
My advice to clients who have windows such as this is to refocus the eye away from the problem. For instance, if the window joinery is a dark colour and the house it a light colour, then consider painting the house in a similar colour to the joinery so it doesn't stand out so much (of course if you weren't planning on painting the outside of the house anyway you could tend to over-capitaise doing this). Another way to re-focus the eye away is by creating some great landscaping around the property (especially the front yard). If you can distract a person enough so that they focus on the landscaping then substandard window joinery my not be so much of a problem.
It's all about really knowing where to spend your money I suppose. If you think changing the window joinery in some way will totally transform the property, then you should do that. Otherwise, perhaps look at other ways to improve it as I have suggested above.
If the joinery has oxodise on the interior as well, then a good way to deal with that is to install blinds in the windows. In particular, wooden blinds or white wooden blinds if that suits the property. These are nice and chunky and modern and will conceal the window joinery easily form the inside. Be sure to buy good quality so that they will last for a number of years (if the property is tenanted especially).
I hope that helps - good luck!