At boot up this morning all my desktop icons has bunched together - so much so that the accompanying text was obscured, even if I made them larger which rendered them useless.

A reboot cured the situation but it's a bizarre thing that I haven't encountered before.

I wonder if any of you have any thoughts as to what caused it?

Cant welcome you, since you been around a while, so Ill star with a hope I can help

There are two causes for this a change in screen resolution or malware/virus.
First if the disarray bothers you, this software will do the trick
Stardock.com - Your Edge In Software

If you are concerned about the problem, I would:
Run a full scan antivirus.
Download and make a full updated scan with malwarebytes
Check the driver for your monitor and graphics.
Run an SFC scan.
If everything is negative, it was probably a one shot deal, with a multitude of reasons (but most likely the reason were caused by change in screen resolution)
Thanks for asking us for help, after all the help that you have given others.

Thanks, already did all that and your last suggestion seems the one, a one shot deal. Simply a glitch.

It was just that I hadn't seen it happen before and thought it was rather bizarre.

I understand where you are coming from The most important thing at this point is to make sure it is not malware or Virus.
I think it was a small problem with the resolution, make sure all applicable drivers are up to date.
Glad that I could be here just to reassure if nothing else.

I recently had problems updating the nVidia drivers and had to roll back, could be a connection there. It's not malware, am absolutely sure of that.

When it was in that state I checked the resolution and the settings were normal and everything except the icons was displaying as it should.

I'll run a full scan using MalwareBytes Free again just in case.

If not malware, its a resolution problem and the driver can definately be the cause. If you were using a new driver I would be fairly certain that the driver was the primary cause. Since, you are using a driver that has served you faithfully for a while, by the symptoms, I do not think its the driver, but it is a definite possiblity.
I am trying to give you the same advice that I would take, in your situation. If it were mine, I would do nothing, unless it happend again. If it did, I would follow this article, as my first step.
Laptop Screen Resolution Problems | eHow.com

Thanks guys.

WinTidy, will also help prevent the problem
icons moving
WinTidy 2.0: Download It Here - WinTidy 2 | PCMag.com
http://www.howtogeek.com/howto/windo...windows-vista/
This indicates that it could be registry, but again, I doubt it in your case
http://aspencomputerservices.com/blo...ve-this-a-try/

Also, if you choose to align to grid the problem should come to an end.

Thanks. I think it was a one off. It was more a problem with desktop icons wrongly displaying as they were actually covering their accompanying text and if you clicked one nothing would happen as several highlighted at once.

I think it was simply a one-time glitch in the desktop rendering engine if there is such a thing.

I've always had Align to Grid checked but not Auto Arrange.

Put it down to the weather...might as well!!