My thanks to you all for the advice and comments.
The 24/7 question was born out of desperation, and with a touch of frustration (that on/off button !)
On Wednesday last i had two tries at getting the pc up & running...
The morning stint was well over 4hrs, and the evening one near 7hrs - eventually gave up & went to bed!
That was the worst day so far - though most others aren't much better, and i just have to leave it off.
Yet today it only took 15 minutes...what a pleasant surprise!
But all the problems are still there.
--
Earlier in the week i unplugged the broadband whilst the pc was running to see what would happen when i reconnected.
(was only off for a few minutes)
Zone Alarm said something about a new connection, but otherwise the pc worked as it was before the disconnection.
(i.e. with the usual problems - and one i forgot to mention, can't open anything in Word or Excel)
Although i did check all the internal connections, and they were all ok, ah'm still a bit feart to leave it running at night.
I somehow don't think i'd sleep soundly knowing a wonky computer has been left to its own devices!
The company i bought the computer from are now out of business, and i only had a set-up chart of where to plug in the cables etc.
It didn't come with any real in-depth documentation - just general stuff & a helpline number.
So can't now update any drivers - not exactly sure what they are, and so wouldn't know how to do it anyway!

Oops, my error, it's actually a near all white screen (not blue) that starts - with the aforementioned improper CPU speed.
And i was wrong in saying an anti-spyware programme deleted something - it was a registry cleanup one.
Not sure what its name was (possibly RegCleaner?) as i deleted the whole thing a while back in case i used it again in error!
So far too late to undo what it did - and what i did.
I think it might be best for me to buy a spare copy of XP for the future too as i doubt my old census CDs will be compatible with Vista.
Or if i do have to use Vista, perhaps install a 2nd hard drive with XP for them alone?
I've now managed to copy & print my e-mail address book - but there are many i don't recognise...
Seems i've forgotten who i've written to over the years!
I've always used Norton - just because it came pre-installed.
Read many times that Norton slows computers down - seems reliable though.
But even so, perhaps worth thinking about AVG.
I do turn off any auto settings whenever i have a choice,
and my CDs have "take no action", but still the Windows installer wants to install...!
But only on some CDs or programmes; not all of them - some work as normal.

--
Anyway, the bottom line is that i'm just going to hopefully "play safe" and go for a new pc - sometime in May probably.
That is when the council will be doing planned renovations for my house; don't want a new pc sookin' in a' the stoor!
So fingers crossed the old one will occassionally deign to start over the next month or two.
As has been said, although a 5yr old computer isn't "old", i just feel it now has far too many problems,
and may be cheaper in the long run buying new than trying to have it repaired.
And hopefully i will then be able to use the external hard drive i bought in Aug 2006 (as discussed on TS!).
Seems it needed a power unit for it to work on my present pc - and i've had trouble finding one!
Or it needs USB2 - at present i've only got USB1.
Or it's faulty, but hope not as it's now well passed the guarantee date! [-o<
I'm obliged to everyone for going out of their way to help, it was much appreciated.
Gave me plenty to think about.
Cheers - Jack
