ACR 6.7 and ACR 7.1 RC can open XSi CR2 files at least on Windows 7. I just checked.
However, on Windows, at least, the DNGC 7.1 RC has a quirk in that when you want to convert a folder of CR2s to DNGs, you have to click the Select button when the folder is highlighted in the file selection dialog and the folder listed in the location area of the file selection dialog. You cannot click INTO the folder and then click Select, which sounds like what you are doing. If you do click into the folder and click Select it’ll say there are no compatible files that exist. I think previous versions of the DNGC would recover properly if you clicked into the folder before clicking Select.
Since your files cannot be opened in ACR, either, then perhaps they are corrupted. You might post a download link to one of them in a message here so others can have a look. If you don’t have a large file hosting service, then set up a free account on www.dropbox.com or www.yousendit.com or similar, upload your file, there, then post the public download link, here, in a reply.
According to the Photoshop CS5 system requirements, CS5 supported 10.5.7 and up. So, your system 10.5.8 is supported.
How are you getting your raw images onto the HD? Directly from the camera or via a card reader? How are you trying to open the images? From Bridge/Mini Bridge or directly from Photoshop or the finder? If from Bridge have you tried purging the cache from that folder? In Bridge under Tools/Cache select Purge Cache.
I'll try the drop box idea later, thank you!
Yes my system should be supported according to all logical reasoning that I have considered, so I'm not sure why it isn't working.
I transfered my raw images onto my laptop's HD directly from the camera with the USB cord. In the past I have done it both this way and with a card reader, both of which would work successfully each time. I attempted to open the images from Bridge (not mini)... Double clicking it told me Photoshop could not open it. Right click - open with Photoshop, same thing. Right click - open with Bridge, still could not open it. I'll try purching the cache folder, thank you. What should that do to help, in particular?
If you have the software CD that came with your camera, install the Digital Photo Professional (DPP) and try opening one of your CR2 files with it. If the initial part of the file is intact, then the thumbnails and previews in DPP may look ok, but if the file is corrupted, when you try to save a JPG from the CR2 it’ll give an error, so try that if things initially seem ok, otherwise.
If DPP needs updated for you OS, then the update can be obtained, here, which I found by searching the USA Canon site for XSi and selected the search-results that include Drivers and Software:
Yes, this I tried and for some reason every time I attempt to open DPP it crashes and says there was an unexpected crash and asks if I'd like to relaunch or report. I've tried both numerous times. When I have more time I'll try downloading the updated version and see that link you posted. Thank you