If you ordered this with an XP downgrade (XP installed), then it will only boot to XP and the only way to use Vista would be to install it.   I don't believe Dell would configure a "dual-boot" system from the factory for you, so in order to have both accessible, you would need to install Vista.  However - and someone correct me if I'm wrong with the downgrade rights - I don't believe you are allowed (by the downgrade license) to have both installed.  If you are though, dual-booting XP and Vista is fairly simply when XP is installed first.  You just need to create some extra room on the hard drive (Unallocated Space) to put Vista.

However - and someone correct me if I'm wrong with the downgrade rights - I don't believe you are allowed (by the downgrade license) to have both installed.

You are correct. The license the purchaser gets is one to use Windows XP until the switch to Vista. You are not allowed to have both active at the same time.

You should open boot.ini filer whic his your windows drive and in this file you can edit some boot options as time for choosing OS and the default OS at boot of your system

For one, Dell would not have shipped a dual-boot system, so editing the boot.ini file wouldn't yield any other valid boot options.

Also, the problem with editing the boot.ini file, is that you cannot boot Vista from XP's boot manager.  You must use the Vista boot loader to boot to one or the other on a dual-boot system.