I have a 100x100 sprite on the screen and I'm blocking out the yellow with this code:

Dim key As DDCOLORKEY
key.low = &HFFFF00
key.high = &HFFFF00
spriteroot.SetColorKey DDCKEY_SRCBLT, key

everything works fine, but when I was testing my code on other peoples computers, there was one computer that showed the yellow that is supposed to be transparent. I tried switching the key and artwork to have bright purple (HFF00FF) and on the trouble computer it only made every other vertical line on the purple part transparent. This is really weird, does it have something to do with differences in video cards? How would I code a transparent color key to assure that the color is correctly blocked out on all hardware?