If you have an XTi older Canon FD lenses will be very hard to get working. The Canon FD mount has a shorter flange focal distance, so any FD->EOS adaptor needs to have optics inside. Canon made a very expensive FD-->EOS adaptor for long tele lenses, but very few were made. You can get third party adaptors, but the quality variation is huge.
However, you can get simple mechanical adaptors that will allow other mounts to work on EOS. Nikon F, Pentax M42/K, Olympus OM, and some other older mounts will work on EOS. However, these lenses are completely manual, meaning you need to stop down and focus manually. Some adaptors have AF confirm contacts that will allow your XTi to show the AF confirm dot, but other than that almost everything must be done manually.