A .XtoDVD file is a ConvertXtoDVD Project File. ConvertXtoDVD is software from VSO Software that allows you to convert multiple video formats into DVD files (VOB, IFO, BUP). To load this project file, click File --> Project --> Load Project, or tap CTRL + L.
Please note that you must have all the files from your project (video, audio etc.) stored in the same place as they were when you saved the project file. So if you got this file from a P2P network or somewhere, it is pretty much useless to you unless you have the actual files included in the project.
Related software tools
VSO ConvertXtoDVD (Shareware)
Easy-to-use tool that allows converting any video file format (DivX, AVI, WMV, ..) to a DVD-compliant video.