Are these the same exact files you've opened before that you cannot now open?
Or are they newly copied files (camera or CF card to computer)?
Can you not open ANY Canon 450D files, even those you find online and download?
What I'm getting at with these questions is to try to eliminate or suspect data corruption of the files on your disk.
Exactly what versions of the various tools do you have, specifically (don't guess - go check)?
I have not been able to successfully open these files in the past. I have been able to open the same file format taken from the same camera opened with the same computer, OS, and programs in the past. I shot these photos with both RAW and Large JPG and I am able to successfully open all JPGs of these same images, just not the .CR2 files.
I have Photoshop CS5, Adobe Bridge CS5 and the Camera Raw Plug-in that came with said Bridge.
Your files are probably corrupted. Please upload one of them to www.dropbox.com or www.yousendit.com and post the public download link, here. If the files you already have are not for public viewing, then take one more of a blank wall or other innocuous subject, transfer it to your computer using the same process as the most recent ones that do not work, and upload that as the example, after verifying it also has the issue.