Kind of a tough call. You can't reproduce anything obviously recognizable and call it yours without permission. That said, though, you can't copyright a chord progression or a picking pattern.
Remove the vocals from Twist and Shout, and you have LaBamba.
Remove the vocals from When I Come Around, and you can do Good (Better than Ezra). Now slow it down, and you can sing Glycerine, by Bush.
Take Bullet with Butterfly Wings (the part near the end where he just keeps going "Despite all my rage, I am still just a rat in a - Despite all my rage....") and remove the vocals. Now sing any of the following:
-Piece of Mind by Boston
-867-5309 Jenny
-Machine Head by Bush (the chorus)
Take Jailbreak by AC/DC and remove the vocals. Now sing Gloria by Them (Van Morrison)
The Cult re-wrote She Sells Sanctuary by putting a new melody and new lyrics and called the same music Love Removal Machine.
Take Everything Zen (the verses) and remove the vocals. Now sing Rockin' In the Free World by Neil Young.
Tonnes of examples....
However, if you SAMPLE the riff from, say, Walk This Way and try to use it as the basis of one of your own tunes, then you're in trouble. Err on the side of caution. AFAIK, there is no definitive answer as to "how close is too close?" Sometimes the courts have to decide that.
Chris