Setting this up is one of the first few things I do after a fresh windows install.
Fireup regedit.exe and head to:
Then tinker with the following (full details of all those regkeys here):
|Bit||UI Setting||Default Value||Meaning|
|0||Active window tracking||0||Active window tracking disabled.|
|Data type||Range||Default value|
Specifies time (in milliseconds) that the mouse needs to remain over a window before the window is activated.