that's a default if your system isn't joined to a domain, or if you don't have it set so that users have to hit ctrl+alt+del to login. If you go into safe mode you can log in as the main administrator. Hwoever, it's a security feature to not encourage you to login as administrator ;) Whenever possible, use a Power User account rather than administrator, it limits the damage that destructive software can do to your PC.
DMA