Co.K of the 21st SCV was organized in Timmonsville, SC in what was then Darlington District (now Florence County). It's first Capt. was James Owens, who was a merchant there and commander of the local milita unit called the "Timmonsville Minutemen". Most of that unit became Co E, 8th SCV, but Owens reorganized another company. They were accepted into state service on Dec. 21st, 1861. Immediately, the regiment was ordered to Georgetown, SC for garrison duties. Co. K became an active Confederate Army unit on Jan. 1st, 1862.
Hagood's "Memiors" is an excellent account of the brigade that the 21st ultimately wound up in. The other book in question is William G. Robinson's "Back Door to Richmond" (LSU Press, 1987)which details the Bermuda Hundreds Campaign that the Brigade took a big part in. You may also wish to read "Gate to Hell" (Stephen Wise, U. South Carolina Press, 1994)about the defense of Charleston, in which the 21st SC played another important role. Also "Confederate Goliath" by Rod Gragg covers the Battle of Ft. Fisher.
Looking at the regiment's rolls, I see both a Samuel and William W. Windham in Co. K. Samuel is listed to have died of disease on Sept. 14th, 1862 in Georgetown. He must have suffered for a while unfortunately, because the regiment left Georgetown in April 1862 for Charleston and he was left behind (there was a small hospital there, ran by Dr. Theodore Dargan of Darlington). I have access to the unit's muster rolls, but I haven't gotten to the "W"s yet.
Hope this helps.
Commander, Pee Dee Rifles Camp
SCV Florence, SC
P.S. I too had ancestors in the 21st SCV in Companies D, E, and G