not sure if this is a camera question or a computer one

My D80 images have just changed from numbering DSC_xxxx to _DSC.xxxx after the first 100 or so downloads. Almost exactly the same thing happened to previous Canon 350D except there it was IMG_xxxx to _MG_xxxx

Anyone know what might cause this and remedy? (no conscious alterations to any software settings......oh er except to make NX default opening prog for .NEF files, but can't think why I even had to do that)