PC stuck Preparing to Standby

My PC is stuck: the screen displays the the blue Windows saying "Preparing to Standby..." as it has for the last 4 hours

The first symtoms of trouble were that no open windows (as displayed on my screen) would respond to mouse (left) clicks (I tried my other mouse, but that had the same result); the mouse ointer was able to move around the screen; minimised windows would open and display on the screen, but there was no response after that. No window had the highlighted blue bar across the top of the screen to indicate the it was the active window. I tried ALT-Tab on the keyboard a few times but that seemed to have no effect - next thing I know, spontaneously,Windows was Preparing to Standby, despite my control panel order the Standby and Hibernate should NEVER happen.

First, what should I do i mmediately ? It seems to me that the only thing I can do is power down (how ? Front panel button, PSU switch, or mains cable, and what order to try them if the preceding one doesn't work ?)

Second, what xaused this ? How can I avoid it again ? (the day before I had stopped a few processes/events on an external HD using procexp.exe and ejected it safely)

Dumb_Question
29.November.2012
Compaq Presario S5160UK (DT261A, Celeron 2.7 GHz) - MSI MS6577 v2.1 - XP Home SP3 - 1.5GB RAM (1GB + 512MB) - Nvidia Geforce 6200 graphics card 256MB on board RAM

 

Thanks again, plodr.

This sends right back to my post about ejecting a permanently busy external USB hard disk....and my posts about restarting my PC (that can of still unsolved worms)

Dumb_Question
29.November.2012
Compaq Presario S5160UK DT261A under XP/SP3
Processor - Celeron 2.7 GHz
Motherboard - MSI MS-6577 v2.1
RAM - 1GB + 512MB (1GB +1GB max) DDR PC2700
PSU - Octigen 300W model 10270PSOTG ('upgraded' from original Bestec 250W PSU [in 2011?])
Nvidia GeForce 6200 graphics card in AGP slot