Here is what forum member LennyRhys said at the official Crysis forums:

"If you tweak the configuration files in
\CVarGroups\
by copying and pasting the 'very high' settings (1st paragraph) IN PLACE of the 'high' settings (last paragraph) the game will load the highest possible settings even though the drop-down menus display 'high.'"

"The difference between 'high' settings and the tweaked settings is immense: shadows are deeper, more realistic; the leaves have better reflective properties, better textures; the colours are better; and the level of detail is simply stunning."

" So I'm happy. I have XP and I'm playing the game at settings higher than DX9 allows (strictly speaking)."

All replies to the topic seemed to confirm Lenny's story (while attacking Crysis and Microsoft for disabling those features for no reason but forcing Vista down the players' throats).

PS I did try it and it works !! Thanks LennyRhys !!