Dear Sir, ('Hi' by itself is ok in an informal email, in a letter or more formal email it's not really acceptable)
I am a qualified C#.NET programmer with over 13 years of experience. (avoid too much repetition of words)
I have the necessary skill to complete your job within the shortest possible time.
I have received great reviews (don't put 'tried' in here - if you have great feedback/reviews say so, if you don't then leave it out) everywhere (either everywhere here or always at the beginning of the sentence, not both) I have worked, and I always provide 100% satisfaction to keep my clients happy. Since your feedback is very important to me, I aim to provide you with the highest quality service.
I look forward to hearing from you. (more positive - no ‘if’)