Hi Doc, My understanding is that chord sequences can't be copyrighted so there is no problem using the chords. Melodies can be copyrighted, so an exact copy of a melody could be problematic. Likewise, lyrics can be copyrighted so an exact copy of a song's words could be a problem.
But really there is only a problem if you try to make money using somebody else's copyrighted material. So, if you are writing a song for your own pleasure there is really no problem. However, if you actively seek to profit from someone's copyrighted material then they may have a legal claim to royalties or damages.
There is one big exception to all this - a parody of a copyrighted song is allowed. For example, Weird Al Yankovitch's satirical send-ups of popular songs are not infringements on the original author's copyright. Although these parodies may use the same melody and lyrical meter as the original song, these humorous creations are legally considered to be original works.
"That darn Pythagorean Comma thing keeps messing me up!"
[url]http://en.wikipedia.org/wiki/Pythagorean_comma[/url]