Ofcourse there isn't a dual boot screen... Vista didn't even know about the 160GB drive when you installed it, so how could it reference ya WindowsXP install?
The way u've got it installed, you will have to use your BIOS to select 1st boot device to choose which Harddisk to boot off.
You may be able to edit the boot.cfg/ini file or whatever Vista uses to add the WinXP partition to it... But i don't know enough about doing that so i'll stop there...
JayMan