I ran into the same issue with Installing Oracle 9201 on a Windows XP Service Pack 2 machine.
I found that when I logged into windows, I had logged into the domain in cached mode. Two things fixed this problem.
1. Make sure I was plugged into LAN instead of wireless after reboot so I get an IP prior to logging in (dell wireless utility doesn't start prior to login). This allowed me to pull down my domain policy stuff and subsequently allowed Oracle to install.
2. Make the change in the SQLNET.ORA file to remove the NTS line. You can also change it to (NONE) if you don't want to comment it out for whatever reason.
CODY Computer Services