Took me less than 24 hours of owning Windows 7 Home Premium before I figured out a way to crash it.

This is a hard lock and requires power off to restore.

What you need:

2 x external usb disks

1 x very large file (I used a dvd iso file for testing) located on your desktop

Insert the two usb disks.  It doesn't matter which goes where or what drive letter they are assigned, however, you need to know what drive letters are assigned to them (My Computer)

Open up Control Panel - Administrative Tools - Computer Management

Browse to Storage - Disk Management

Start to copy the large file to either of the USB disks.

Before the file copy completes, change the drive letter of the 2nd usb disk to any unused drive letter (make sure you change the drive letter of the disk not being copied too)

The file copy will come to a screeching halt, even though it is being copied to the usb drive that didnt have the drive letter changed.

Within about 20 minutes, explorer will stop responding.

CTRL + SHIFT + ESC to open task manager

Attempt to kill running processes, even attempt to kill explorer.....

The system is hard locked and totally unresponsive.  CPU Usage will be near 0%  On my system, memory was at about 2.6GB out of 8GB.

Eventually even the start menu stopped working.  Only recourse was to power cycle the system.

On reboot, I was able to duplicate this repeatedly.