Why is the Holy Spirit often referred as "Ghost"? Why is the Holy Spirit often referred as the Holy "Ghost"? Does not the term ghost usually used in a negative connotation that relates to "scaring away"?
Basically, language changes over time.  The Holy Spirit is referred to as the Holy Ghost in earlier English translations, such as the KJV and the Douay-Rheims.  At that time, the word "ghost" seems to have had the same connotations as "spirit" does today.  

So, it was a great translation then, but today the word "ghost" is too closely associated with the ghosts depicted on Halloween and such.

The original languages certainly did not intend to associate the Third Person of the Trinity with such images, so "spirit" is a better word in our language today.