Is there a program that can make my mouse in Windows XP act like the mouse in Solaris (e.g. the window the mouse is hovering over is activated without bringing it forward).
The X Mouse gizmo is just what the doctor ordered.
If you had more of a registry hack in mind...
Windows does support focus-follows-mouse, though there is no GUI configuration exposing it. Instead you must edit a registry key and then log out and back in for the change to become effective. You can use regedit to edit the key.
On Windows 2000, XP, or 2003, you need to change a binary-valued registry key:
This is a little-endian bitmask. For focus-follows-mouse, add the flag 0x1. For example, my XP SP2 laptop originally had a value of 9E 3E 05 80, which is 0x80053E9E. To activate focus-follows-mouse I changed to 0x80053E9F, or 9F 3E 05 80 in regedit.
According to http://www.winguides.com/registry/display.php/18/ you can also achieve raise-on-focus by adding the flag 0x40.
TweakUI has an option for that.
Mouse > X-Mouse, and there you have settings to activate on hover (or even raise).