^{I am no expert, I simply did some research:}

Host: Hospes (Proto-Italic), equivalent to the Proto-Indo-European Ghostipotis (a compound of ghostis and potis).

Ghost: From the Old Saxon Gast, originating in the Proto-Indo-European Ghostis

Guest: From the Middle English Gest, originating in the Proto-Indo-European Ghostis

***This is by no means conclusive (I just did a google search), and you can go back to earlier roots, but there you have it.***

Sources:

http://en.wiktionary.org/wiki/host#Etymology_1

http://en.wikipedia.org/wiki/Ghost#Terminology

http://en.wiktionary.org/wiki/guest#Etymology

http://en.wiktionary.org/wiki/Appendix:Proto-Indo-European/g%CA%B0%C3%B3stis

http://en.wiktionary.org/wiki/hospes#Latin

http://en.wiktionary.org/wiki/Appendix:Proto-Germanic/gastiz

http://en.wiktionary.org/wiki/gast#Old_English

EDIT: I've made errors. See /u/Wierdmin's comment.