Possible to detect Windows desktop style?

I need to pop up a modal JS-based dialog (for some reason can't use
popup window, much less so showModalDialog()), and I'd like to imitate
the system popup titlebar according to user's desktop settings
(classic, XP, Vista).  No big deal to make it work, but how do I know
the style to display?  Any chance to figure out these settings
programmatically either on the client (preferrable), or possibly in
asp.net from something like Request["ServerVariables"] (not the best
thing but will work for this project).  Would prefer cross-browser
compatibility, but can use IE-specific solution (sorry, no client side
activeX may be used, just plain JS/DOM/DHTML).  Asking the user to
manually select visual preferences is not an option.

Thanks

NNN