Install window on a portable ide harddisk

Question

i have hp pavilion 2601tu laptop.i have a portable USB Seagate IDE 160 GB harddisk.Boot priority i have set like this.........1.CDROM2.USB Hard Drive3.NoteBook Hard Drivewhen i install windows xp on this portable harddisk.When all the files from Windows xp CD copied into the seleted partition as in usual installation.As u see that after coping the files system restarts and after that remaining installation of OS occurs.The problem with me is that when laptop restrat after coping files it shows a msg------Unable to load Operating System...Plz Help Me......For further information plz mail [e-mail]