Yes, just plug in the PCI card, they usually come with four ports. XP will probably recognise it, but you may need to add a little software that will be supplied with the card.
You could also consider an external hub that will plug into one existing slot, they come in all different sizes.
I have four at the back; two at the front and four on my monitor which plugs into one of the rear slots, giving me a total of nine.
Used thus:
1) Printer
2) Scanner
3) Joypad
4) Joystick
And I plug my camera in and out of a front port.
I'm still using the PS2 sockets for mouse and keyboard.