Fastest, cheapest, easiest way to decorate for Halloween or a Halloween Party:
Drape white sheets over all the furniture and windows. It looks like a haunted house and a much more realistic and fun decoration than store-bought paper decorations. Add candles in old fashioned holders on top of covered tables, and a few hidden flashlights to provide creepy uplighting.
Aside from being a great atmosphere for Halloween, and being free or cheap - It's also great for party preparation. No need to worry about old furniture or party cleaning, etc. - just cover it all up with sheets.