In Windows 7 my administrator account has somehow got set to 'guest' so I have lost all administrator access rights on my Dell laptop

To boot into safe mode, follow the below steps:

 

1.      Remove all floppy disks, CDs, and DVDs from your computer, and then restart your computer.

Click the Start button, click the arrow next to the Lock button, and then click Restart.

2.      Do one of the following:

·          If your computer has a single operating system installed, press and hold the F8 key as your computer restarts. You need to press F8 before the Windows logo appears. If the Windows logo appears, you will need to try again by waiting until the Windows logon prompt appears, and then shutting down and restarting your computer.

·          If your computer has more than one operating system, use the arrow keys to highlight the operating system you want to start in safe mode, and then press F8.

3.      On the Advanced Boot Options screen, use the arrow keys to highlight the safe mode option you want, and then press ENTER. For more information about options, see Advanced startup options (including safe mode) .

4.       Log on to your computer with a user account that has administrator rights.

 

When your computer is in safe mode, you'll see the words Safe Mode in the corners of the display. To exit safe mode, restart your computer and let Windows start normally.

 

Refer the below link for more details:

 

Start your computer in safe mode

http://windows.microsoft.com/en-US/windows-vista/Start-your-computer-in-safe-mode

Method 2 :



If no joy with Safe Mode :

Try this - use the Hidden Admin Account to lower your account to user (to lowest level) APPLY/OK 
then go back and reset it to Admin APPLY/OK - this helps clear corruption. Do this a few times.
Then fix the password in your normal account. Be sure to leave your account as Admin if you
wish.

Make another Admin Account with your password and use it to fix the others when needed. (just to
repair with, not to use as regular account, a safety valve) Always keep a spare ADMIN account.




Try to Enable Built-in Administrator Account
First you’ll need to open a command prompt in administrator mode by right-clicking and choosing “Run as administrator” (or use the Ctrl+Shift+Enter shortcut from the search box)

Now type the following command:

net user administrator /active:yes

You should see a message that the command completed successfully. Log out, and you’ll now see the Administrator account as a choice. (Note that the screenshots are from Vista, but this works on Windows 7)

You’ll note that there’s no password for this account, so if you want to leave it enabled you should change the password.
Disable Built-in Administrator Account
Make sure you are logged on as your regular user account, and then open an administrator mode command prompt as above. Type the following command:

net user administrator /active:no

The administrator account will now be disabled, and shouldn’t show up on the login screen anymore.


Method 3 :



If the above methods fail, you may have a corrupted User Profile and may want to fix it so try fixing corrupted user profile using the below link
http://windows.microsoft.com/en-AU/windows7/Fix-a-corrupted-user-profile


Regards,
Azeez Nadeem - Microsoft Support