Windows 7 by default doesn't enable the Administrator account any more, so using a blank password on Administrator is out (this design feature is probably a good thing for the majority of cases).
After a bit of searching around the internet I found this article:
It was very helpful, and worked perfectly.
The only thing I would add is that if you don't know the username of the account you want to change, at the very last command prompt before you reset the password simply type "net user" and it will list the user accounts on the machine.
For my sake:
Let's get Started!
1) The Boot Priority in the BIOS needs to be set to boot first from the optical drive; insert your Windows disk and re-start the PC. At the first screen select your language then Repair your computer then it'll do some searching, at System Recovery Options dot Use recovery tools then select the OS and click next; while there make note of the OS drive letter, it may not be C: like mine is D:.
2) At the next screen select "Command Prompt" then in the Command window type "regedit" (without the quotes) and hit enter.
3) In the Registry Editor window that opens click to select "HKey_Local_Machine" then at the "File" menu select "Load Hive".
4) In the "Look In" in the "Load Hive" window that opens navigate to (in my case) LocalDiskD:\Windows\System32\config\SYSTEM and click "Open".
5) In the "Load Hive", "Key Name" box give the new key a name e.g. " 123 " (it can be whatever you like) and click OK.
6) In the left pane of the registry Editor window click the " + " sign to expand the "HKey_Local_Machine" key then click the " + " sign to expand the "123" key then click to select the "Setup" key.
7) Now in the right pane right click "SetupType" and select "Modify", set the "Value data" to " 2 " and click OK.
8) Again in the right pane right click "CmdLine" and select "Modify" and set the "Value data" to "cmd.exe" and click OK.
9) Now just as a precaution, close the Registry Editor at the "File" menu click "Exit"; then in the Command window that's still open type "regedit" and hit enter to open the Registry Editor again.
10) In the Registry Editor click the " + " sign to expand HKey_Local_Machine and click to select the new "123" key, at the "File" menu select "Unload Hive" and click "Yes" for the "Are you sure ..." question.
11) Now close the Registry Editor at the "File" menu click "Exit" then in the Command window type "exit" and hit enter to close the Command window, then remove the Windows disk and restart the PC.
12) When the PC restarts a Command window will open, in that window type "net user