the 1/8 th jack in the back of ur comp is stereo, or at least should be, if u look at the jack that plugs into the comp u will see that it has a piece of platic part way down the shaft, that is where it seperates the right from the left on the stereo part, ur guitar amp should only be mono, so u have to us the adapter so i will play on rite and left channels at same time, i dont know what u have for stores over there, but here we have
radio shack stores and the people there can help u fig it out
chuck