Boot screen is the one which occurs in the startup of windows.The image that is displayed during the boot is hidden away in a system file
called ntoskrnl.exe. This system file is loaded during the system boot
and is what displays the boot image and animation.
BootSkin by Stardock, is a free application. This program is very easy to use and is also very safe. It is highly efficient when compared to modifying the system files your self. You can get a copy here.
Tune Up utilities is another program which can be used for changing the boot screen.It has a tune up styler program .This is a shareware program , you can get demo version at www.tuneup.com.