PCI and PCI-E are completely different busses. Different signaling, different protocol, different everything. You can't get a "cable" to go from one to the other. Startech does, however, make a low profile card that goes into the slot first, and another card plugs into it. It's a weird bastard adapter, but it works.
PCI-E slot to PCI card -http://www.startech.com/Cards-Adapters/Slot-Extension/PCI-Express-to-PCI-Adapter-Card
and PCI slot to PCI-E cardhttp://www.startech.com/Cards-Adapters/Slot-Extension/PCI-to-PCI-Express-Adapter-Card
Weird, yes, but it is possible. It takes active logic and switching though. Not just a cable.