US Robotics USB Modem (External)

Hi !
I’m using a US Robotics External modem with USB port. now a strange one, if I shutdown my computer and then restart nothing appears to happen. I just get that blue screen with Compaq (motherboard)logo.It seems as if BIOS is not checking devices or loading OS.also i changed boot order still it happening.However if I unplug the modem before booting everything is normal. i’m using winxp pro as my o/s.