Here's a quick tip that comes in handy fairly often for me. I use it most after someone has forgotten their password and my usual method of removing their user accounts password has failed (linux boot disk and editing the hive, which is quite a bit more complex and will be outlined in another post) but can be used at other times too. Read on for detailed information on enabling and disabling the administrator account.
The account is created in Windows 7 or Vista but not actually enabled. To enable, 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) and type in:
net user administrator /active:yes
Technically, you don't need to run that in a command prompt, but you wouldn't see for sure if it worked or failed as it will automatically close the program after running it whereas the command prompt will stay.
So now you can log out of the current user and back in as Administrator, no password is required even. Now you can do whatever you needed to and then disable the account again as it's not recommended to run under that account all the time.
To disable, log out of the Administrator account and back in as a user. and open an administrator mode command prompt as above and type:
net user administrator /active:no
Personally, I have a USB drive with a bunch of utilities and scripts such as this command on it to speed up common things like this or resetting tcpip stacks or clearing the upper and lower registry entries.