I opened a new document, started typing until 100 words or so ... then the
whole screen ---- the typed words, tool bar and taskbar suddenly turned
blurry and everything hung. Cursor also went missing. I have no choice but to
reboot the computer.

Retried but same thing happened. Sometimes the screen just turns blank (no
image at all) instead of getting blurred and hung. In both cases , I need to
reboot.

What could be the cause of such problem? Thanks.