R help - Oct 2008 - spec.pgram help?

Hopefully this will not seem too ignorant of a question. I am having a hard time
picking out the sources of the differences between:

abs(fft(x))^2/length(x)

and 

spec.pgram(x, taper=0, log="no", plot=FALSE)

Also from the limited testing that I have done since the DC
"frequency" is not returned from spec.pgram how can I tell what has
happened to the series when I specify demean = TRUE or detrend = TRUE?

Any help?

Thank you.

Kevin