Reboot and go into BIOS - usually the Delete key on a Dell, I believe - and look for a setting that allows USB Legacy Support. Enable that if it exists and that should give the external drive the power it needs for the system to recognise it. While you're in BIOS, check that the system can allow an external USB device to be bootable - if it does, your external should show in the list. You might need s second reboot into BIOS to do that bit.
I recently tried to install windows 7 on my dell inspiron 1100. My dvd rom don't work, so I used an external dvd writer. Now i'm trying to reinstall windows XP and the laptop is not reading from th external dvd drvie.
What should I do?