2006-11-13: I have completed customization of my desktop OS. It now displays custom boot image, custom wallpaper (both at logon and after), custom logon dialog, custom theme, desktop sidebar.

To customize your Windows XP there are several different areas you can work on.

Boot Screen

Boot screen can be created, edited, and replaced with UserXP's utilities - Logon UI Boot Randomizer and Boot Editor.

Logon Screen

There are two types of logon screens - Desktop and Network. Read how to fix each one of them below.

Desktop Logon

Logon Loader or UserXP's utils can be used to change desktop logon screens.

Network Logon

Network logon screens are contained in msgina.dll in \Windows\System32 folder. Use Resource Hacker or XN Resource Editor to extract and replace resources within the dll file. Dialogs, text, images can be customized. Msgina.dll then needs to be replaced in safe mode or use Replacer and reboot.

Themes

Custom themes can be applied by replacing uxtheme.dll with a custom one. After that it will be possible to use custom themes and styles for Windows XP. Theme settings contain visual styles, backgrounds, sounds, etc.

Customizations can be downloaded from deviantART.

Styles

Visual Styles are subsets of themes and can be applied separately. They include setting different UI properties. The file extension for them is (usually) .msstyle and many can be found and downloaded from the net.

Use Replacer to replace UXTheme.dll and then apply themes (read below).

My customizations

Here you can find my customized images. These include boot bitmaps and network logon bitmaps. The original image, used as a base and wallpaper, is DragonStyle from dragonkahn.

Vista

If you want to make your XP look like Vista, install some custom theme (Vistab2), put custom background (dragonstyle), install Desktop Sidebar and that should be it.

Read Sticking with Windows XP in a Windows Vista World for a list of Vista-like applications available for XP.