Anyone knows how to remove "Genuine Windows Validation" message from my system?

When I had bought this PC, my machine came with windows XP installed.

I wasn't getting any such message.

Some time back, the motherboard of my PC stopped working and the dealer installed a different one on my PC.

He also reinstalled XP on my PC.

Now I keep getting this stupid message in the lower right-hand corner of my screen:

You may be a victim of software counterfeiting. This copy of Windows did not pass genuine Windows validation.
how to get rid of it?