"Boot Screen" = ntoskrnl (and some others) " Logn Screen" = LogonUI You can modify it with ResHacker! A littel search sould help you to find some more answers!
Upload it on here: http://virusscan.jotti.org/ (den gibt es auch auf deutsch: http://virusscan.jotti.org/de/) It's a online virusscan with 20 different scanengines... ... and you will see it isent a virus!
Or better for you the Black Version : http://www.zedox.co.uk/xpizedarkside/ You need only to patch the "LogonUI.exe" and "masgina.dll" (or manually with ResHacker) Or in other words: There is no way to change this with a normal "theme".