If the saved videos work in Media Player then the problem could possibly be something to do within the application that you are using to read the mail message.
Is it a Browser that you use to access your Hotmail, or the Hotmail plugin or is it some other email package?
If its an Internet explorer browser, try using a different browser such as Opera or Firefox.
Just in case you are experiencing any potential file system errors it might also be worth checking for have any bad disk sectors / clusters and repairing.
Try running a system CHKDSK command that will check the Hard Disk for any errors and fix them automatically. You can also run an additional SFC command that will check the system integrity and report any further errors if they exist.
- To run CMD command click on the Windows Start button and type in the search box CMD.EXE
- right-click on CMD.EXE filename found and Select 'Run as Administrator'
- the Elevated Command Prompt window should pop up
- At the Command prompt, type CHKDSK C: /R and hit the Enter key
- Windows will report that the drive is locked, and indicate that CHKDSK will run at he next system reboot - hit the Y key, and then reboot.
- CHKDSK will startup automatically on reboot and will take some time depending on the size of the drive, so just let it run until complete!
After the CHKDSK has completed, Windows should boot normally (possibly after a second auto-reboot)
- now run the SFC command as per instructions:
- Click on the Windows Start button and type in the Search box CMD.EXE
- right-click SFC.EXE filename found and Select 'Run as Administrator'
- the Elevated Command Prompt window should pop up
- At the Command prompt, type SFC /SCANNOW and hit the Enter key
Wait for the scan to finish - make a note of any error messages and then reboot.
Worth a try, which at least will eliminate or identify any Hard Disk errors