I've never had this problem, but research suggests that dxtex.exe is related to DirectX which Windows uses to process complicated graphics. I would make sure that your DirectX software is up to date and functioning properly, and if it seems to be, then see if you can re-install dxtex.exe. This software I believe should be free.
If that still doesn't work I would get some tech help, I don't think this issue is related to WinMip itself.