I suspect this is a Windows XP restriction rather than an Outlook 2000 one.
Make sure your user account is labelled "Computer Administrator".
If that's not the issue, and you're using XP Pro, it may have email restrictions set in the Group Policy Editor (not applicable to XP Home).
See here:
http://www.theeldergeek.com/group_policy_editor.htm
I don't know if NIS has any relevance here, but you could do worse than getting rid of it anyway. It's a memory-hog and and it's virus-detection capability has recently been branded as "scandalous" and "a rip off" by a national PC magazine, easily outclassed by - wait for it - the free AVG anti-virus. (Computer Buyer - Feb 2007)