whoa, i totally don't remember talking about rock, big, or distant. must have been drunk.
Anyway, if you really want to waste the time to try to make a fairly realistic guitar riff from a sample, the only way I know how to do it is in a tracker-type program with sequenced filters. If you have some other audio program (don't think fruityloops would do it, don't really work with much more samplewise) where you can make filter settings every step of the song, this will work there too. It's still not perfect, because a guitar does not work like a sampler in the slightest, but it's better than nothing.
You sample your guitar: play an A-form powerchord around the 8th fret. This is the region of the fretboard with the most medium tone quality. From here it's easier to fake the tonal quality of lower and higher notes than anywhere else on the fretboard. If you hang a PC mic over your amp just above the center of the speaker and keep the volume pretty moderate--not loud enough to clip the mic, just loud enough to drown out the noise, it'll sound pretty good.
How to get a crunchy ass metal sound (in case someone actually doesn't know). You got to use overdrive and maybe a little additional distortion. If you have a tube amp with built-in drive and it's got good saturation, perfect, otherwise I recommend a DOD overdrive plus. DOD pedals are really crappy, but the sound is just right. Get as much gain as possible. If it doesn't break up, just distorts, you have good overdrive. If it doesn't quite distort, push your amp as far as it'll go. Get all the pregain you can. If it still doesn't distort, put a distortion pedal before your overdrive, or scrap this whole rig and just use a boss metalzone, but that's not the old school way of doing it, which is better. Your amp eq should have the lows on about 7.5, the mids all the way down (that's the important part), and the highs between 5 and 10 (the closer to five, the better it'll sound when you resample it at higher pitches). If you have a presence or shape knob, you want it pretty low (not the best for live metal, but better for sampling). Use the neck pickup of your guitar.
OK, you got a guitar sample. It should be a pretty long, held-out powerchord. Now,
In your sequencing program, you have to worry about three things: the pitch, the volume, and the lo-pass filter. The volume and lo-pass work against each other, so the lower one is, the higher the other one should be to keep the levels even. You can normalize the whole riff later to get it as loud as you need, but you want it the whole thing about half volume for sequencing. Lo-pass settings: get as much resonance (Q) as possible without it squelching. The cutoff is dependent on the pitch of your guitar sample. To figure out about what cutoff is good for what pitch, play the sample at a high-E, volume all the way up, and turn the cutoff down until it sounds about as loud as the sample would sound at half volume with no filter. Remember the cutoff frequency here. For a low-E chord, the Q should still be all the way up (always all the way up), but the cutoff should be up at 20khz, or off. A B-flat should be the average of whatever the cutoff for a high E and a low E was (e.g., if a high-E sounded good with the cutoff at 10Khz and the low-E was at 20Khz, then the cutoff for a B-flat would be (10+20)/2=15.).
So to find the good cutoff for every note in between, keep interpolating in this fashion (to find the cutoff for a note exactly between two other notes you already know, add the cutoffs for these two notes and divide by two).
So every note should have a volume and a filter cutoff setting, which you figure out as above and write down to remember.
Palm Mutes: I find that sampling a palm mute and trying to use it at different pitches doesn't work very well no matter what I do. Palm mutes work funny. They have weird tones and they're very dependent on time, and they're so short that time stretches would ruin them. So the next best thing would be to take the original sample of a held-out power chord and put a filter envelope on it with a sharp attack-decay spike and a sustain level of about 75Hz, with a release time about twice the length of the attack+decay. This will do decently.
This is all still gonna sound a little fake, but it's the most rugged way of doing it and usually sounds better than any other one-shot sample method. If you're lucky you can get a hold of a guitar sample with unique tonal qualities that will sound good at every pitch without all that filtering, but I have yet to figure out how to make one on my own. But it probably involves a B.C. Rich, which have the flattest, most uniform and characterless tones I have ever heard out of a guitar, and an off-brand solid state amp with built in distortion (yeah puke). So of course it wouldn't be the best guitar soundwise in the world, but it would be easier to work with.
But like I said, this is really complicated and takes a lot of time, and you're really better off just sampling whole riffs.