I have now run the MS Memory Tester and Memtest and Memtest86 (as Carol suggested), and it's finding no problems at all.  I switched the memory sticks over, but the problem persists - I'll follow your lead and take both out next, Mouser, and then replace.  When I ran Everest, I noticed that there seems to be a difference between the modules, as you'll see below (sorry for including this in the post, since it's pretty long and bulky).  This info means little to me - but does anything leap out at a more savvy reader?


   [ DIMM1: Micron Tech. 8VDDT3264HDG-335C3 ]

    Memory Module Properties:
      Module Name                                       Micron Tech. 8VDDT3264HDG-335C3
      Serial Number                                     111DB16Eh
      Manufacture Date                                  Week 41 / 2003
      Module Size                                       256 MB (2 rows, 4 banks)
      Module Type                                       Unbuffered
      Memory Type                                       DDR SDRAM
      Memory Speed                                      PC2700 (166 MHz)
      Module Width                                      64 bit
      Module Voltage                                    SSTL 2.5
      Error Detection Method                            None
      Refresh Rate                                      Reduced (7.8 us), Self-Refresh

    Memory Timings:
      @ 166 MHz                                         2.5-3-3-7  (CL-RCD-RP-RAS)
      @ 133 MHz                                         2.0-3-3-6  (CL-RCD-RP-RAS)

    Memory Module Features:
      Early RAS# Precharge                              Supported
      Auto-Precharge                                    Not Supported
      Precharge All                                     Not Supported
      Write1/Read Burst                                 Not Supported
      Buffered Address/Control Inputs                   Not Supported
      Registered Address/Control Inputs                 Not Supported
      On-Card PLL (Clock)                               Not Supported
      Buffered DQMB Inputs                              Not Supported
      Registered DQMB Inputs                            Not Supported
      Differential Clock Input                          Supported
      Redundant Row Address                             Not Supported

    Memory Module Manufacturer:
      Company Name                                      Micron Technology, Inc.
      Product Information                               http://www.micron.com/products/category.jsp?path=/DRAM

  [ DIMM3: Kingston K ]

    Memory Module Properties:
      Module Name                                       Kingston K
      Serial Number                                     18FFFFFFh
      Manufacture Date                                  Week 29 / 2004
      Module Size                                       512 MB (2 rows, 4 banks)
      Module Type                                       Unbuffered
      Memory Type                                       DDR SDRAM
      Memory Speed                                      PC2700 (166 MHz)
      Module Width                                      64 bit
      Module Voltage                                    SSTL 2.5
      Error Detection Method                            None
      Refresh Rate                                      Reduced (7.8 us), Self-Refresh

    Memory Timings:
      @ 166 MHz                                         2.5-3-3-7  (CL-RCD-RP-RAS)
      @ 133 MHz                                         2.0-3-3-6  (CL-RCD-RP-RAS)

    Memory Module Features:
      Early RAS# Precharge                              Not Supported
      Auto-Precharge                                    Not Supported
      Precharge All                                     Not Supported
      Write1/Read Burst                                 Not Supported
      Buffered Address/Control Inputs                   Not Supported
      Registered Address/Control Inputs                 Not Supported
      On-Card PLL (Clock)                               Not Supported
      Buffered DQMB Inputs                              Not Supported
      Registered DQMB Inputs                            Not Supported
      Differential Clock Input                          Supported
      Redundant Row Address                             Not Supported

    Memory Module Manufacturer:
      Company Name                                      Kingston Technology Company, Inc.
      Product Information                               http://www.kingston.com/products/default.asp

Okay, I'm onto something here... I've taken out the 512mb memory stick that I bought a few months back (on eBay, dammit, so probably no chance of a refund), and replaced it with the original HP 256mb stick, and the problem doesn't happen.  What I realised was that it was managing to select about 4 screens worth of text before switching off - so I tested it with a PDF document, and lo! it did the same thing.  I'm just downloading Picasa again to try that and see if it works...  Can anyone advise me on how to test the memory for physical or other errors, or is there a chance I can just wipe it down, put it back and it'll work properly? (Naive about memory, it seems.)

Suddenly this evening my computer has started doing something very strange.  While working in Word, my girlfriend highlighted a large block of text, moving the mouse down to expand the selection; at a certain point, the machine blinks off - powers down.  It's repeatable: I've done it three times to try to see what was happening, but I can't figure it out...  I had a similar problem with Picasa, which actually made me stop using it, though I like the program a lot: as it scanned through my images as it started up, the computer would suddenly shut down.  In 25 years of working with computers, I've never seen anything like this before... 

I run a 2.66gHz HP Pavilion ZD7010 laptop, with 768meg ram.  I have various programs running in the background, including: NOD32, Outpost Pro, RegDefend, TrojanHunter, WindowBlinds, ObjectDock, WindowFX, TopDesk, Google Desktop, AdMuncher, The Bat!, CalendarScope, WinPatrol, ProcessTamer, Konfabulator, MS Antispyware.

Has anyone ever seen something like this?  Do any of the programs listed above suggest areas for me to investigate?

Thanks for any help...