Depends what you want to preview. Bolter and Chainsword have several virtual painter programs you can make use of.
You can find one for a Fire Warrior, a Chaos Marine and an Ork >
HERE <
And ones for a Space Marine (and Terminator) and a Sister of Battle >
HERE <
I'm pretty sure there used to be an Eldar one on the GW page, but I'm also pretty sure it's not there any more. You might be able to dig around and find it, somewhere, though.
Actually, a quick visit to google did it for me; an Eldar (and another Tau one) >
HERE <
Also, on one of the pages that led me to that Eldar painter, someone suggested the customisation thing in the Dawn Of War game. I've never thought of that before, but if you happen to have the game that might be an idea, too!