Prepare Your Computer
Because Micro$oft wants you buy everything from them, Windows XP wants theme files to be digitally signed. If they aren't then XP won't let you use the themes. You must first patch your uxthemes.dll file in your C:\Windows\System32 directory. This can be easily done using the Uxthemes Patch which you can download
here. The patch supports Windows XP, SP1, SP2, XP 64 bit and Windows Server 2003. Run the patch, it will "fix" your uxtheme.dll file so you can use 3rd party visual styles. Disregard any warnings about files being changed, that is what we want to do. After you run the patch reboot your computer. Note: We recommend you create a restore point before running the patch just for good measure. We did not author this file we have used it our selves with no problems, use it at your own risk.
Install the XP Theme Files
Themes are usually downloaded as a .zip file which must first be scanned using a virus scanning program, then unzipped to a temporary folder you can create on your desktop. Inside that folder you will most likely find a wallpaper image, .msstyles file, a .theme file and possibly a font file. it all depends on what the author has chosen to include in their theme package.
If your theme package contains one or more font files (.ttf) copy them to your C:\Windows\Fonts\ folder.
You may also find one or more .theme files (like axis.theme. Copy it/them to your C:\Windows\Resources\Themes\ folder. You will see a folder called Luna already in there, leave that alone, it is your existing Micro$oft XP Theme.
Locate the .msstyles file, create a folder in the C:\Windows\Resources\Themes\ folder, I usually name it what the .msstyles file is named (like axis.msstyles so the folder would be axis).
If you find a file called ShellStyle.dll, create a folder called Shell inside the first folder you created. Create another folder inside that called NormalColor. Copy ShellStyle.dll into the NormalColor folder.
Most of the time you will unzip the file and everything will be properly ordered and only need to be copied to your C:\Windows\Resources\Themes\ folder.
To install the wallpaper copy the wallpaper image file to your My Pictures folder.
How to Use the XP Theme
While in your My Pictures folder that contains your wallpaper, click on the wallpaper file once to highlight it. In the menu on the left side of the window you will see at the top Picture Tasks. Click on Set as desktop background. You should see the wallpaper on your desktop.
If the wallpaper needs to be stretched to fill the screen close the My Pictures window. Right click on the desktop, select Properties. The Display Properties window comes up, choose the Desktop tab. Your wallpaper should be highlighted in the list, using the Position box choose if you want it Tiled, Centered or Stretched.
While in Display Properties if your theme had a .theme file select the Themes tab and select your theme from the list. If your theme only had a .msstyles file go to the Appearance tab and select your theme from the Windows and Buttons list.
After you are done making your choices you can go to the Themes tab and save your theme by clicking Save As..., and then name it something.
That's all there is to it and all without buying any programs.
How to install themes in xp. 
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