Chances are, the problem is not in the OS. MPEG4 has different implementations with different feature sets, and the compressed stream can be placed in different file containers. The actually used format is the combination of stream formats and container formats. To play a wide range of formats, you need to install a set of codecs. Almost nothing is bundled with XP be default; so one would need to install a set of codecs covering all formats which you can come across when you download clips from Internet or get them from other sources.
One of the way to get all at once it to use the free product called K-Lite (Mega) Codec Pack. See:
http://en.wikipedia.org/wiki/K-Lite_Codec_Pack[
^],
http://codecguide.com/[
^].
You need to pick the package according to the version of Windows and instruction-set architecture.
Usually, installing it all by default is good enough.
As Laxmikant_Yadav recommended, you might need to install the latest DirectX. This is always a good idea. In fact, at the end of installation of K-Lite (Mega) Codec Pack you will receive a warning if your version of DirectX is not compatible. In this case installing it is a must.
—SA