new external hard disk not detected

i have new external harddisk (250G) but my laptop unable to detect it.iam using xp but the settings of my laptop is vista(a friend of mine did that).i'm thingking is that possible why my new harddisk is not detected because the setting of my laptop displays is vista?i don't know what to do. my ext.hardisk not recognized in the disk management.but in other pc (XP alone) it works only in my laptop not work.do i need to reformat my laptop into XP alone? thanx for the answers..