The MsiEnumRelatedProducts function enumerates products with a specified upgrade code. This function lists the currently installed and advertised products that have the specified UpgradeCode property in their Property table.

private static extern UInt32 MsiEnumRelatedProducts(string strUpgradeCode, int reserved, int iIndex, StringBuilder sbProductCode);

while(0 == MsiEnumRelatedProducts("{FFFFFFFF-FFFF-FFFF-GUID-STRINGFFFFFF}", 0, iIndex++, sbProductCode)) {

Documentation

[MsiEnumRelatedProducts] on MSDN

Access PInvoke.net directly from VS: