Raw files (CR2) - Cannot modify IPTC Core fields, get error messages

Question

I really hope someone out there can help me out here because I'm at my wit's end!!!!
I have a bunch of raw files in a folder, these are just pictures I took for a bit of general practice over Christmas.
Now, whilst trying to modify the IPTC Core Fields (basically adding my name and my email address to the pictures), it worked fine for most of the pictures, but for some reason it will not work for the last 20. This is driving me nuts. :(
The first picture which doesn't work was taken literally 2 seconds after the last one which does work. All pictures work perfectly well in Photoshop, I can open them, modify them, save them as jpgs... no problem. But it just won't let me change the metadata. The pencils on the IPTC Core fields are nowhere to be found, the option File>File Info appears greyed out... etc.
The error messages I get:
1. When clicking on any line of the IPTC Core Fields (such as "Creator") =
i "This property cannot be modified".
2. When clicking on the keywords tab and trying to enter keywords =
i "The file Name.CR2 cannot store XMP metadata. No changes will occur"
I have scoured the web for help, visited multiple forums... to no avail. It looks like other people are having the same problem but no-one seems to have a solution. I haven't done anything different to those 20 pictures, so I just can't think of a logical answer, it just doesn't make sense!!
I am only starting to catalogue my pictures in the hope of going professional one day and this is extremely discouraging... if anyone out there can help, I'd be ever so grateful. Right now I just feel like pulling my hair out!! :'(
OK, rant over :)
Thanks very much for reading and thanks very, very much in advance for any suggestions/ideas!!

Answer 1

I have run into the same problem. I downloaded photos in raw format ".CR2" from a Canon 40D via a compact flash card reader to my laptop. I labelled the ones I wanted green and then transferred these images via wireless transfer to my desktop. I had not checked any keywords while these were on the laptop. I used the Bridge (CS3)File/Copy to menu selection to effect the transfer. Both systems are Windows XP, SP2.
I then tried to check a keword in my list for an image and got the error message number 2 reported by Maria Atkins above. I got the same for all images on the desktop system.
Observations:
-the green labelled items are correctly filtered on the desktop, but the labels on the images all show white.
-not all images even have .xmp files associated with them. It appears to be a random thing.
-although I have checked my preference to have double click open an image in Camera Raw in Bridge it will not do so with these images, always opening Camera Raw in Photoshop.
-the camera exif data is correctly attached to the images, whether there is a .xmp file associated or not.
-On my laptop I found I can check keywords and they are stored with the image with no complaints from Bridge, and whether the image has a .xmp file associated with it or not. This is after I had copied the images to the desktop. I was just checking whether the problem was with the original images or just the transferred ones. It appears to be that something was not done correctly in the transfer.
This is not a trivial issue since it is essential for cataloging images to enable future searches. I hope someone from Adobe is paying attention and will respond with an answer.