thanks for pointing to this (its in the MS link ).

vbSystemModal 	4096 	System modal; all applications are suspended until the user responds to the message box.

VbMsgBoxSetForeground 	65536 	Specifies the message box window as the foreground window

there is mandatory some difrences in messagebox  modality capabilities.One suspend all apps and wait to the user feed and other is on foreground.

i always used 4096 with no problem.originally i used a fullscreen app and a stupid messagebox stayed on background blocking all the process and wainting for confirmation....