Quote : "PCI Express, officially abbreviated as PCI-E or PCIe, is a computer expansion card interface format introduced by Intel in 2004. PCI Express was designed to replace the general-purpose PCI expansion bus, the high-end PCI-X bus and the AGP graphics card interface. Unlike previous PC expansion interfaces, rather than being a bus it is structured around point-to-point serial links called lanes."
They look different and are not interchangeable in any way.
Standard 32-bit PCI slots are generally white in colour and around 3 inches long.
PCI Express has two types of slot, a long X16 slot for graphips (which to make matters worse may occasionally operate at X8 speed) and a much shorter X1 slot for general expansion cards.