My HDD have an DELL OEM version of windows xp installed, now if i plug the HDD into other brand of motherboard. Will it boot as usual ?

even if it will boot,its considered illegal

i believe your OS will crash due to the different settings your hdd had .. you don't change a OS HDD to another mobo unless it is the same mobo .. else you're basic disk will turn into dynamic disk ~

It might or might not boot as usual, if you're lucky and the drivers are semi compatible it'll boot, and then b**** at you with a ton of warnings. If you aren't lucky, it'll BSOD before getting into windows.

i think you will have problem bro. i have done and always came annoying BSOD..

It will not work. Your windows has been configured with all the drivers for the previous motherboard. unless your new motherboard is the same brand and version as your old motherboard

There's still a way to boot the pc from HDD. Just make sure you uninstall ALL drivers like the printer, Nvidia/ATi, on-board audio, etc, before plug into another motherboard.