You do it in a series of cut and pastes.
Go to the first post you want to quote. Click on Quote. When in the reply dialogue box, "copy" the quote including the [quote] [ /quote] (not sure how you do it in Vista, I either right click to get a menu or use my browser's edit function). Either click on back or just close the reply dialogue box. Go to the next post you want to quote, click on Quote. In the reply dialogue box, the 2nd quote is already visible. Put your cursor before the 2nd quote [Quote] thingy (I like to put a line between the two by hitting enter) and "paste" (again, I'm not sure how Vista works but I have to either right click or use my browser Edit function) your first quote in place. And so on and so on.