The above link is to change the Logon screen.. Which is OEM customizable by default.
To add to cybercore's post, use ResHacker to modify the 1033 (?) string of that file, You'll have to make a new copy of it and then replace it.
NOTE: THIS DOES NOT WORK WITH VISTA SP1. IF YOU ATTEMPT THIS IN SP1, YOU WILL NOT BE ABLE TO BOOT YOUR COMPUTER.
If you are a desktop customization kinda guy, one of the first things you’ll want to figure out is how to create a customized boot logo. There’s a little utility that you can use to create the boot logo formats. DO NOT DOWNLOAD THIS UTILITY IF YOU HAVE VISTA SP1
First you’ll need to download the Vista Boot Logo Generator 1.1 utility, which we’ll use to create the correct logo image type. Once it’s installed, you need to make sure that you run the application as administrator:
We should choose an appropriate logo file. Since I’m a big fan of linux, and not as big a fan of Windows Vista, I chose an image of Tux making windows suck that I found over here.
Choose two versions of the image, one needs to be 800×600 at 24 bit color, and the other 1024×768 at the same color depth. Both files must be in Bitmap(BMP) format. Save the file somewhere like your desktop, because we can’t yet copy directly to the correct directory, until we take ownership of the file.
To take ownership of the file, you’ll have to open an administrator command prompt. (type cmd into the start menu search box, and hit Ctrl+Shift+Enter), and then run this command:
Now copy the file that you created into the C:\windows\system32\en-US\ directory. Make sure you choose the overwrite option.
Now you’ll need to use the msconfig utility. Just type msconfig into the start menu search box, and then click the Boot tab:
Check the “No GUI boot” option, and we’re all ready to reboot and test it out.