When you first had the windows installed, did they open and hold up correctly, or have they been crooked from the get-go? If they held up correctly, you probably just need new window balances. I pulled up some information on the right side of this page for you to look over. I think this will help give you a better idea of what you can do. Replacing balances is very easy and you can definitely do it yourself.
If they were crooked from the start, then they were installed improperly and will need to be reinstalled. This may be a bit of a project, in which case you would want a professional to help out.