There are 5 different groups with this name:

1.) Halloween, a French Symhonic-Prog-Band. They existed between 1988-2002 and recorded 5 albums
http://www.progarchives.com/artist.asp?id=143

2.) Halloween, an obscure 70's Disco-Band. They only recorded one album, "Come See What's It All About".

3.) Halloween, an American Horror-Metal-Band, that exists since 1985.
www.halloweentheband.us

4.) Halloween, a Portuguese Rapper, in the business since the early 90s.
http://www.myspace.com/halloweenyk

5.) Some songs recorded for the Walt Disney parks have "Halloween" as artist. These songs are made for the theme in October and November.