I have a windows xp disk but am uncertain if the computer that we just bought can be dowgraded. It came with no install disk and it istalled itself , presummabley from the harddrive, as soon as powered on for the first time. Format disk and install XP, if so how and what will we do if we want Vista later since there is no install disc when we bought the new COmputer. Computer is an Acer. Thanks.