Missing desktop and taskbar - worm or windows update? Trying various fixes and reporting back

Update: fixed
Here are the instructions that someone for whom I shall shortly be baking cakes suggested:

Open regedit
See bit in pink below on how to access programmes with no desktop or toolbar, after the 'new task' bit type in regedit.

go to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon

Find the Value “Shell” it should just say Explorer.exe No path or anything else just Explorer.exe. If it doesn’t change it to Explorer.exe and reboot.

The instructions worked perfectly :)

Disclaimer: boy can you asterisk things up if you make a mistake with regedit. I speak from embarrassed experience.

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If no desktop, taskbar (ie no Start button) press Ctrl+Alt+Del to bring up the Task bar. Choose the Applications tab, then New task... and browse to find a copy of Firefox or Safari (I seem to have no IE available at the moment, this may be important) to do web stuff. All programmes are accessible here using this method. There's also a Shut Down option on that C+A+D when you've finished.

My computer is misbehaving. It's an Acer Aspire One running Windows XP.

I switched it on this evening and, while booting up, it suddenly told me that Windows Malicious Software Remover Tool had removed a worm of some sort. I'd never met WMSRT before so at first I thought perhaps that was something a bit dodgy and closed the window, so I'm not sure what it was trying to tell me. The McAfee thing that comes bundled with my BT account suggested all was well... but the little green line that tells you how things are with your processor kept topping out at 100% (to see this press Ctrl+Alt+Del then click on the Performance tab to see how things are going).

Yesterday I downloaded Evernote - I downloaded the same programme at work with no problems. Didn't visit any dodgy websites, don't remember any worrying popups appearing, don't think I downloaded anything else...

I do remember that at switch off the computer wanted to download some Windows Updates, so I let it do that. The next time I switch it on - problems.

Windows Updates have caused all sorts of problems before and they usually fix themselves in the next day or so, but this is the first time my computer's been ostensibly useless.

What's happening?

• At switch on the computer boots up as far as the 'apply wallpaper' stage, no icons appear, no taskbar; the desktop is completely absent.
• Pressing Ctrl+Alt+Del has been a bit of a help (it's let me open Firefox and Safari - curiously Internet Explorer is nowhere to be found, this may be significant and suggests there has been a worm) - once you've C+A+D click on the Applications tab and then the New Task.. button and then Browse.. to find the relevant file. I used this to open the browsers.
• I don't know what the address for the desktop is (it's called Explorer I think, but typing just the word 'explorer' into the box brought up Windows Explorer with all the folders and files hierarchies. Not what I'm after. This is the address for Firefox, for example C:\Documents and Settings\All Users\Desktop\Mozilla Firefox.lnk - no idea if clicking this will work of course (and it would only work if you also had a shortcut link to Firefox on your computer).
  Edit: Duh. MSIE is actually iexplorer.exe not explorer.exe - but even then that didn't work until I tweaked regedit.
  
• Nerds have previously wrestled with a similar problem but I've not quite worked out whether or not the fault is with a worm virus thing or with Windows update. Poster going by the name of Diddly seems to have the exact same problem as me http://en.kioskea.net/forum/affich-71716-taskbar-disabled-explorer-exe-won-t-boot

I am going to try some of the suggestions on the forum, and on Twitter and post here. Hopefully it won't necessitate having to buy a new computer or anything like that ;-)

----- First report

• I tried the very simple seeming instructions suggested by meow in the forum post linked above. Didn't work. I thought perhaps the trick was to close an old programme running and open a new one, but nope.
• Discovered that Windows+R doesn't do anything on my laptop - this might be something that works on Vista or Windows 7 etc., but it doesn't appear to on XP.
• Noticed that I can turn off the computer from the Task Manager (Ctrl+Alt+Del to open up the window) - there's a 'Shut Down' menu above the five tabs, to the right. Especially useful when there's no Start button and the windows key does nothing.
• Loving the nerds on Twitter who leapt to my aid with suggestions - switching it off and then rebooting was a popular suggestion. While it's rebooting you press F8 to bring up some options, including opening in safe mode. I tried this but couldn't get the window to appear but then it was suggested that I should just PRESS and hold F8 while it was booting - that worked. Initially I chose 'open in safe mode' but still no desktop, then tried 'last good configuration' - nope.
  
• Tried another suggestion of running sfc /scannow but needed to get a command prompt on screen to do it (no Windows or Start > Run available) so used Ctrl+Alt+Del / New task and then typed cmd (it means command) and up the C:\ prompt appeared. Faffed about trying to get sfc /scannow to work without much success but eventually it took (I was in C:\Documents and Settings\Jo Brodie and it worked there. I'd previously used the cd.. command a couple of times to get back to the plan C:\ prompt.
  (every now and again I open up the command window and ping servers nostalgically but I haven't really used it in anger since the mid 90s to be honest).
• Finally the computer's now in the process of doing this /scannow and I am using the time to document what I've tried. Not sure what's going to come out the other end of the scan but will report back when I can. Or I might go to bed.
• Ironically I'm planning on going to the Museum of Computing at Bletchley Park tomorrow (prompted by going to OSHUG7 last night). Perhaps I should take the ailing laptop with me ;)
• Still puzzled by the curious lack of Microsoft Internet Explorer - the icon does not appear in the list of shortcut links on my desktop (as accessed via task manager) yet it's normally there. See picture below - it's missing.
  

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