If you are using Windows XP, you probably already know that you cannot install third-party themes onto your computer as your operating system checks them for the digital signature that certifies they come from Microsoft. Consequently, if the said signature is not detected, the theme cannot be installed. This restriction can be bypassed by patching the Uxtheme.dll from your XP with a custom one, and this can be achieved using UxTheme Patch for Windows XP SP3.
As implied by the software name, this patch can be applied only if your XP computer is already running the latest service pack, namely the third one. If you have SP2, you need to use this tool.
First of all, you need to replace the default Uxtheme.dll with the one found within UxTheme Patch for Windows XP SP3. Rather than simply overwriting the file on your computer, it is recommended to use Uxtheme Multi-patcher to make sure everything goes smoothly (it is also advisable to create a system restore point before applying any changes to your system).
Once the Uxtheme.dll has been tweaked, you need to restart your computer to fully complete the patching process.
To confirm that UxTheme Patch for Windows XP SP3 has worked, you can browse the web for an XP theme that you like, download and install it. You can find it in the Display Properties window (simply right click on the desktop, then choose the Properties items from the shown menu), select the newly-installed theme and apply it. Your XP computer gets a new appearance as its taskbar, windows, icons and menus look different (it needs to be mentioned that not all themes modify all these elements). If you are not pleased with the result, you can restore the original XP theme or try a new one.
After you install this patch onto your XP PC, you get the liberty to revamp your computer as often as you like. However, it is best to scan each and every theme you download from the Internet to make sure they are not infected, regardless of the website that stores them.