So, it boots from the hard disk, and you get into Windows, and then you get an error that you have no hard disk that you just booted from? A clean dwm.exe process is the Desktop Windows Manager. However, it also comes as a virus masquerading as that process.
If you're not running Antivirus, I suggest you start (with something like Microsoft Security Essentials or Avira Free). Boot your computer into safe mode and run a scan.