This is not "my baby" but it is a mod that I find necessary to business, and think it should be a part of x-cart by default.
The zip from post #185 is for version 4.1.x (the "Edit $xcart_dir/include/func/func.product.php" is a dead giveaway as it doesn't exist in versions other than 4.1.x) and from my original notes I don't see where any php files are modified anyway.
I have attached the files I used for version 4.0.x (probably version 4.0.19) for you to compare. As I said before this is a old thread, and it has gone through several version of x-cart that are different from one another, so you will have to find where the discussion pertains to your version.
To make matters worse, there are 2 distinctly different mods that pertain to new products, making it difficult if you do a search of this forum. The posts in the thread at
http://forum.x-cart.com/showthread.php?t=1084&page=9 ...... post #90 through about 110 appear to deal with your specific version, and would probably be a good starting point.