Please note that I have tried this out only for XP. They are all FREE.
AlienGUIse
Has 4 Skins to choose from. Very good, consumes very little memory and it looks very good as well. Only Dark Skins are available. You can also separately download skins for Windows Media Player.
Download from HERE
Vista Inspirat 2
Turns your computer to look just like Vista. Its very good and looks exactly like Vista. You can choose which settings you like if you want to reduce on the usage of system resources. Like shadow's and the zoom effect when clicking on icons.
Download from HERE
FlyakiteOSX
Want your computer to look like a Mac, lot of people do so here is a great software to transform your computer to look like a Mac.
Download from HERE
Vista transformation Pack
Another vista transformation pack to make your computer look like Vista. This is very popular and even more popular than Vista Inspirat 2
Download from HERE
These are the ones I have tested and found no problems. There are several skins out there. I havent included any Windows Blinds Skins because you have to pay for Windows Blinds after the 60 day trial.
Remember just give me some more to add if you have tested it on your computer.