Hi and welcome to the forum. I'm not going to be of any help on either of these.
Don't Mess With Me (Temposhark): http://www.youtube.com/watch?v=OMeWTcPOLGg
Maybe someone with a good ear for musical composition can take the combination of the notes being played by the strings (cellos?) and the melody being sung and turn them into chords for you, but I'm not that good.
Diseased (Seether): http://www.youtube.com/watch?v=6MOQOVPXZwg
Sounds like a "death metal ballad", if there is such a thing.
Sounds like the intro and verse center around F#m. I think I hear a progression similar to F#m - Bm - E - Bm. At about the 1:05 mark, the death metal vibe (distorted and muted power chords) kick in. That progression might work for them also.
Best I can do for you. I know there are several members who monitor this forum that have great ears for this sort of thing. Hopefully one of them can give you more help that I can.
"Just because you've always done it that way doesn't mean it's not incredibly stupid." - Despair, Inc.