Found this trick ina an MSDN article among other searches:

go to the loacation of the exe of the program that you want to add the xp style to, create a text file and insert the following text into it:
Code:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity
    version="1.0.0.0"
    processorArchitecture="X86"
    name="CompanyName.ProductName.YourApplication"
    type="win32"
/>
<description>Your application description here.</description>
<dependency>
    <dependentAssembly>
        <assemblyIdentity
            type="win32"
            name="Microsoft.Windows.Common-Controls"
            version="6.0.0.0"
            processorArchitecture="X86"
            publicKeyToken="6595b64144ccf1df"
            language="*"
        />
    </dependentAssembly>
</dependency>
</assembly>
If you want you can fill out the description fields but it may not be needed

Save and rename the text file to appname.exe.MANIFEST (appname being the application's exe file name)

This trick may not work on all apps:
  1. The app may have a few graphic inconsistancies or mess up some widgit
  2. it may look like crap(see #1)
  3. propritary widgets are being used
  4. or the app will just crash out at some point

For reference, in later versions of Nero 5, the CD toolkit (CD Speed, InfoTool) uses these exe.MANIFEST files