Go to your system folder and see what version of user32.dll you have, mine is version 5.1.2600.2622. You should be able to download a newer version on the net. That should solve the problem.
According to the msdn documents SetLayeredWindowAttributes should be available for win 2000 and up. If it doesn't work for you then you need to upgrade your user32.dll.
As an aside, It would be very helpful if people with win2000 do try this script and see if it works for them. I would very much like to make the finished product available for as many users as possible.
If some can get it to work and others can't then we'll know it's simply an upgrade problem. If not then I'll dig into some of my old API references and I'll try to come up with an alternative.