MD5 crack
OK. Download a program to attack it for you. I prefer cain and abel, others prefer john the ripper.
Google one of them and download it. If you're using C&A, click on cracker, click MD5 hashes, add the hash, right click it and press dictionary attack. If that doesn't work, brute force it. However, if the password is too lonng it can literally take months to find it.
There is no way too reverse-engineer MD5 hashes.
Use Cain (url = oxid.it) as it was already said.
And, btw, when you have no md5 crackers, you can … try Google to find the solution. Google is able to find the hash (unfortunately not always with the solution) on some sites. One of them is, for example, http://milw0rm.com/md5/list.php. If Google doesn't find it, use the search engine of the site (milw0rm e.g.)
Hope that helps.
Good cracking :)