Do not put a PCI-e card in a PCI slot. It will not fit and the card won't work. The PCI card has 58 or 60 pins on each side with the slot (62 pins would be present if not for the key notches for alignment). PCIe 16x cards have 32 pins.
There are adapters to put a PCI card in a PCIe 1x slot (will fit any PCIe slot) or a PCIe1x card in a PCI slot. See http://www.amfeltec.com/index.php
for a sample of the adapters. For the PCIe 16x cards, you need a new motherboard.
If you are placing a PCI-e 16x card in a PCIe 16x slot or a PCI card in a PCI slot, then open the side of your case. Locate the matching slot (remove the current card) or an empty slot depending on your motherboard. Depending on the case, you may need to remove a screw to remove the old card. Wiggle the card straight up to remove it. If you are putting in a new card, you will need to remove the cover from the slot. (For some larger PCIe 16x cards, you need to remove the adjacent slot cover as well.) Some covers just pull straight up or pry a lever up and then pull; for others, you will need to use a screwdriver to break the cover out. Make sure there is sufficient room near the slot for the card. (You may need to move other cards around.)
Then line up the card so that the back of the card will fit into the opening and the contacts on the bottom of the card are above the center of the slot. Firmly press the card down into the slot without changing the alignment. You may need to wiggle it (press down on the back and front, alternatingly). Then make sure the card is fully seated. Attach the addtional power cord from your power supply, if needed for a PCIe 16x card.
Close your case. Then if this is a new video card, turn on the computer and enter the BIOS (press F1, F2, F10, Esc depending on the machine). Disable the on-board video. Then reboot the computer and load the drivers you need for the card.
I hope this helps.