Rule Number 1: Always make backups of the original files in a safe place, and work ONLY from copies.

But you knew that.

Rule Number 2: Make your conversion to the "least lossy" format you can open.

If you convert direct from your "unworkable" file format to .jpeg, you're at the mercy of the converter, and you may lose quite a bit of image data. If you can find a converter that will save the files in a "full data" format, like uncompressed .bmp or .tif, you have more to work with in Photoshop, and you'll have better control over the final result. BMP and TIF files can be huge, but you don't necessarily have to keep the intermediates. TIF files can be compressed, but the compression is relatively "lossless" unless you use one of the odd TIF formats (there were about 9 last time I looked). JPG compression removes additional data every time you save, even if you save to the same compression settings.

If you can, you should try to make a "type-for-type" conversion. Corel used a lot of "vector graphic" file types, and if that's what you have you should look for a way to convert them to another "vector" format. If they're a bitmap, make the conversion to .bmp. Photoshop (or PS Elements) can open quite a few of either kind.

Even for black and white pictures, if they were scanned as color you should probably try to convert them as color. Make the change to b/w or grayscale in Photoshop. You'll generally get better results.

The .cpt format isn't listed in any of my graphics file data books, but they're mostly rather old. Of course, if it was a special-purpose format just for a specific proprietary program, that program may be the only "clean" way to open them, although there are a number of "converters" available.

John