Hi,

The ORA 12638 implicates that "Credential retrieval failed" -
The authentication service failed to retrieve the credentials of a User.
It seems that you have a problem with your administrator rights in the specific machine.

Regarding the other question,
Oracle installation creates a group (local in windows NT), which its members can login into oracle without the need of a password.
It seems that the Administrator Account has some problems with security issues.
My advice - try installing windows 2000 from the begining.
If you are using Directory services consult you Sys Admin.

Gool Luck.