See option #3 on this article:
Step #1:
Open regedit, go to HKEY_CURRENT_USERControl PanelDesktop
Edit UserPreferenceMask value to be 9F 3E 07 80 12 00 00 00
[You should only have to modify the first pair]
Also in HKEY_CURRENT_USERControl PanelDesktop
Edit ActiveWndTrkTimeout, change ‘Base’ to Decimal and put in 150 as the ‘Value Data’ [this value is time in milliseconds that it takes for the focus to change.
to avoid some issues with the taskbar I recommended using a value like 150 or so, but if you’d like to have a different delay before the window loses focus, set it to whatever you like.] Step #3:
Log off and log back in
Apparently, you can still use Microsoft’s Tweak UI PowerToy in Windows XP SP2 compatibility mode to set the X-Mouse settings in Windows 7.