Finale notepad:
http://www.finalemusic.com/notepad/It's the free version of Finale (which is expensive) but it'll do most of the basic functions you'll need for writing music. It does tabs as well, with MIDI playback. Limitations include an 8 stave limit per score (you mentioned guitar stuff, 8 staves is plenty unless you're scoring for concert band or full orchestra). You can't really customize things so well as the paid versions. I think you have to register to get a serial number, but just give them a secondary e-mail address (they send special offers once every once in awhile).
Powertab:
http://www.power-tab.net/guitar.phpIs a pure tablature program. It accepts your input as numbers on the tab, and shows them above in standard notation. If you're going to be doing work with guitar, it's useful. It's also useful in that it automatically displays in both tab and standard notation. I have a friend who uses it to convert his tabs to sheet music (which I enter in Finale).
I've had good experience with Finale 2008, but I'm a music student so the $300 (student edition) was worth it to me. I use Finale exclusively, but I score for string orchestra or full orchestra, not guitar. One of my friends writes stuff almost exclusively on guitar (or for other things...but he uses the guitar to write it) and he uses Powertab.
Hope some of that was useful...
