My Experience

Yes, you got the title right. By default Administrator account is disabled. How did I get to know about this? Colleague of mine James Joseph who did extensive research on account and shared it with me. I remember once I even called Lenovo Support to seek help on Administrator account. It looks like Technical guy is not much aware of disable/enable of Administrator account.

So, if you wish to enable it. Here are the steps.
1. You need to have administrator rights account
2. Run Command prompt with Run as Administrator option.
3. Type net user Administrator /Active:Yes and net user Administrator /Active:No to Disable
4. In Control Panel Go to manage users and click on manager other user.
5. Change password for Administrator
6. Log off
7. You should be able to see Administrator login option

If you have any problem in above steps, just Google or Bing for "Enable Administrator user in Vista" ;)

If you know any thing interesting like this please feel free to share it with me.

Have fun :)
Rajesh