Hi guys, thanks for the help, it's much appreciated - I'm not as mac-literate as I should be, brung up on PCs...
Re boot-up, normal except at end, when Finder toolbar flashes intermittently then disappears, and no icons except dock appear on desktop. HD icon is not visible.
Most apps in dock will open and function EXCEPT Finder - toolbar flashes on then vanishes. Some apps 'quit unexpectedly' - eg mail and sherlock, whilst others (iTunes, iPhoto) seem to function fine.
Sorry, not sure what's meant by 'single user mode'...
Re Finder, if I left click on icon in dock, toolbar appears momentarily, but not long enough so far to get the cursor on to it! If I right click, it sez first 'app not responding' then 'Finder not running'. So can't - so far - open Finder menu to get at Preferences.
Re GUI, no I haven't (knowingly) been tinkering nor using shape-shifter - altho did install Design Workshop Lite, a free 3D drawing app, then uninstalled as it seem to be very crashy.
Re archive/install, sorry to be so ignorant, how do I do this?
Thanks again...
Chris G.