I do believe that a chiro can help you with your concerns, however, you most likely will not have immediate improvement. A lot of posture problems are due to laxed and/or spastic muscles. Unfortunately, that is not something that can be fixed with just one or two adjustments. There are natural curves to your spine...for instance, your lumbar (lower) curve, is developed through crawling as an infant. Most likely you have a decrease in this curvature, and an increase in your thoracic curve. It will not feel anywhere normal if you stand up straight, as it takes years to develop this posture. Your chiro can evaluate you and hopefully give you some great strengthening exercises for whatever weak muscles you may have. Adjusting will help with the spastic muscles....
Sorry if this isn't the most coherent post...I have a cranky 1yr old on my lap. And since I have never evaluated you...this is me 'assuming' everything:D