If they replaced your motherboard, they should've given you a CD with it that has drivers and assorted utilities on it. Since USB has been around for a while now, you don't normally have to install USB drivers to get the ports working right, especially in Win XP. However, if your motherboard came with a CD, see if it has USB drivers on it. If so, install those before you take it back.
Good luck, hopefully that'll work. If not, may the techs that "fixed" it get it right this time.