To add to Kirk's advise:
Every device has it's own (specific) profile. Your camera has one, your monitor has one, your printer has one, and finally your file has one too. They are all different. To be able to get consistent results you will really need to calibrate at least your monitor.
Anyway, you said you changed the colour profile to aRGB. My guess is that you (also) changed the profile of your monitor with that. This is likely the cause of the muddy colours. Either calibrate your monitor as advised, or change it back to what it was set to before you made the change (at least this default profile will give you better results than aRGB).
Side note: the choice of profile for your files aRGB in your case is a good one as this has a fairly large gamut. However if you ever publish files on the web, best you convert them to sRGB AND embed the ICC profile so that everyone will get the same colours (Windows or Mac).
Hope this helps,