USB flash connectivity

I installed a new w2k server sp4, all services are working fine (DHCP, DNS, PDC, etc), but for some weird reason, any workstation can’t use memory flash drives… When I connect them, XP seems to recognize them, but the drive never shows up. I am wondering if there are some setting that I don?t know (probably a policy setting) that prevent stations to install new drives, or use flash drives. Does anyone know someone?