PCI Modem

I just bought a 56k v92 pci modem to fit into my dell dimension 3100c. When i tryed to fit it the metal part of the modem was to long to fit in to the case. So i contacted dell and they said they don't sell modems and i should goto the high street and buy a external modem. As i don't want to buy a external modem as they are alot more expensive than a pci modem and i only need it for a couple of weeks. All i want to no is can i get a pci modem to fit my case and also if there is 2 pci slots what would fit in there as the case is so small.