I wish to programatically change the general width of the scroll bars in windows XP from my program.
The idea is when the program window is shown, the scroll bars will increase in width to facilitate the use of a touch screen, but when other applications have the focus, the general scroll bar width will shrink to normal.

Its easy enough to change the registry entry which controls this feature ("HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics\ScrollWidth" and "ScrollHeight") but short of rebooting the machine, I cannot get the changes to take effect immediately.
I am aware that the windows have to be sent a message in order to check the registry have tried various combinations of the following.
This works well if for instance I want to change the icon font size which is the adjacent value in the registry. - SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, SPI_SETNONCLIENTMETRICS, 0&, SMTO_ABORTIFHUNG, 100000, lngResult) - but cannot get it to change the scroll bars.
I suspect the answer lies in the value used in the 3rd argument.
Appreciate any help or suggestions.