My camera (a Canon EOS Rebel XT/350D) shows error cf. I've tried all the suggestions in the manual to fix the problem. I've bought new CF cards. Nothing makes a difference. Any help would be appreciated.
My guess is that you have a broken/bent pin in the CF card slot. Given that the Rebel XT/350D was introduced in 2005, and that any copy you have is liable to be well over a decade old. That's really old in dSLR terms. And that CF card slot has a lot of small delicate pins in them that can easily be damaged by carelessly jamming something into the slot, or even simply mis-seating the card.
I'd suggest inspecting the slot with a flashlight to determine whether or not there's any physical damage. If there is, then it's probably time to consider getting a new camera, as servicing the camera is likely to cost far more than the camera is worth, given that a service call to Canon tends to flat rate around US$300, and a used XT goes for less than US$150 these days.
I removed the card from its place inside the camera. Using my household vacuum hose I ran it across the holes of the card. I put the card back in the slot, turned the camera on and it continues to work as if it was new. It's worth a try at this point. Too expensive to have my local camera shop look at it with a $100 base look at it bench charge.