You'll need a dedicated raster application to open and edit them.
Try IrfanView, it will at least open them. Then you can copy and paste into Xara Designer Pro, but you won't be able to save out as DDS.
Probably better to use PhotoShop after installing the DDS plugin
► http://3dg.me/2d-graphics/nvidia-plu...shop-x64-64bit

Certainly isn't a Web chat related question though