We had already shared many stuffs like Mac OS X Lion skin for Windows 7, Windows XP, and now its time for LogOn Screen.
Now you can get Mac OS X Lion logon screen on your Windows XP, df006 at DA has created it for XP users,
First download the RAR file from below link and the extract it either using WinRar or 7-Zip (Download it from Recommended Section) and then first apply the font and the apply the user pic which come in the package.
And at last you have to replace the LogonUI.exe, present in %windir%System32 with you get in downloaded package, you might not be able to replace it, you can use Replacer for it, it can be found in Recommended section.