Buy a legit copy of windows.
A year back I've modded my asus bios with SLIC to activate windows, everything seems to be fine throughout that time.
After installing windows essentials recently (needed windows movie maker), I occasionally get "this copy of windows is not genuine" watermark.
The only way to remove it is opening System, blam! the watermark is gone. But it's only temporary and watermark comes back again.
How do I keep the watermark from showing up?
Buy a legit copy of windows.