Guest user account disabled

You can modify your accounts using 'Local Users and Groups':

Click 'Start'.
Enter 'lusrmgr.msc' in search field and click the top entry.

'Local Users and Groups' will open.
Select 'Users'.
All local users will be listed.
Click on 'Guest' and then 'More Actions'
Select 'Properties'
From the 'Guest Properties' window, select the 'General' tab and uncheck the 'Account is disabled' box.

OK.
To get the 'Group Policy Client Service failed logon' error does suggest to me that the service is running but it is still worthwhile checking the status just to be sure.

To check the state of the service:

Click start button and, in the 'Search programs and files' box, type 'services'.
Near the top of the list that appears, click 'Services'.
Services will open.
Look down the 'Name' column for 'Group Policy Client'.
In the 'Status', 'Startup Type' and 'Log On As' columns, can you confirm that they show 'Started', 'Automatic' and 'Local Service' respectively?

(Note that you should not be able to make changes (greyed out) as the service is protected by the OS for security reasons.)

I note from an earlier post of yours that you had issues with 'Update.dll'. Have you experienced an update failure? I ask because it has been reported that such a failure can cause registry corruption with some Group Policy keys.

I assume you are part of a workgroup and not a domain.
If that's the case, try the following:
Log on with your administrator account and create a new account with administrative permissions.

Change the workgroup to anything else (it doesn't have to exist). You can do this through Control Panel > System and Security > System.
Now click 'System protection' in the left pane.
System properties will open. Click the Computer tab.
Now change the workgroup name and click OK.
You should see a welcome box. Clear it and restart your computer.

Log on with the new administrator account you created earlier.
Create a user account (*not* with admin privileges).
Try to switch to new user account. If successful, switch back to your new Administrator account and try switching to your Guest/sons account. If this works, it may indicate a problem with your 'normal' Administrator account profile. If this fails, it could indicate a problem with your Guest/sons account profile.

If successful, switch back and reset your original workgroup as detailed above.
Reboot and log on with your normal administrator account and test again. Also test the newly created administrator account.

I'm sorry that a solution hasn't been found.
Probably a reinstall is the best course of action.

Do you have a recovery partition on your machine? You can test with Alt + F10 while booting which would bring up a recovery menu if present.

As you say, you will need to preserve your files etc before you restore your operating system.
You will need to reinstall any applications you have - you will almost certainly not be able to copy them off and back again and have them work.

I don't know which operating system or machine that you have but I would use a USB stick for your backup files if available.

Think carefully about which files you want to preserve. It is so easy to overlook something and then, of course, it is too late. I've done it myself! (Email messages and account information for example.)

If you find that a recovery is not possible, you may wish to contact Acer with a view to purchasing recovery media. I don't know what Acer would charge but it may be more economical than buying a Windows OS from your high street. I would assume Acer media would include all the required drivers. A 'standard' Windows disk would not of course so, if you go that route, I would recommend you download all the required drivers from Acer's download site first.

I would like to offer some advice which may leave you better protected in the future:

When you have installed your new OS, do not use your 'primary' administrator account other than to create your own admin account. You can then use your own admin account to create and administer any other accounts on your machine. In so doing you can preserve your 'primary' admin account as (up to a point) a 'get out of jail free card' for use should you ever need it.

I would also not use the 'Guest' account for your son - give him his own named account'. I suggest that because I have read that Microsoft do not recommend the 'Guest' account be disabled which suggests it may have other uses.

Finally, I do hope you are not too discouraged! Remember that the Acer forums are still in their infancy. As more members come on board and the knowledge base broadens, things can only improve. Your particular problem is one of the more difficult that I have encountered. I have researched this and found that, while not too common, it is far from unique. Unfortunately, while I have read many suggestions, I have yet to find a definitive solution. This may well explain why, on this occasion, you haven't had the response that you perhaps had hoped for.