Where Can I Download The File Mdismpx.sys?

If Mdismpx.sys file is related to windows then the file got corrupted you have to reinstall your windows. If this file is related to some software then reinstall that software. You can download lots of softwares for free from the following website

www.download.com

Hope this might help

There are many websites that are providing windows free files and dll files because sometimes you purchase windows that may not give you complete files so you can check this website and down this file for you.
d3dx9_43.dll