The paint program that came with P4, if it is the same version I have, will not run on XP or Vista. It was made for Windows 95, and when I try to run it on XP it fails.

The other options are:

Argile - about $100 - can do diffuse and bump, no specular or displacement channels:
 
http://www.n-sided.com/3D/argile.php?rub=1

3D Coat - about $120 - can do everything:
 
http://www.3d-coat.com/

Body Paint, ZBrush and Mudbox - more professional, can do lots of things other than paint, but this comes at a higher price and a steep learning curve.

There are a few other options, like Blacksmith 3D, which I haven't tried.

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