Most are safe (no malware, won't harm your computer, no scam) and quite a lot will be free. I don't have any personal experience with any of them, since I never bought something in the iTunes shop, so don't have any .aac file around.
If it doubt, you can try to find them on a trusted download site (like our download.com, tucows, brothersoft etc) and see if there are reviews there.
This is a search on our site: http://download.cnet.com/1770-2025_4-0.html?query=convert%20aac%20mp3&rpp=10&searchtype=downloads&filter=licenseName%3D%22Free%22|platform%3DWindows&filterName=licenseName%3DFree|platform%3DWindows
It gives much more programs than applicable, but the top one, for example, looks good to me.