There is little difference between local administrator and power user in Windows NT/2000/XP.
I wouldn't say that.
Power Users cannot install/remove device drivers
Power user cannot install activeX controlls
Power users cannot take ownership of system files
Power users cannot change networking settings
Power users cannot change (or even READ) security policies/IPSEC policies
Power users cannot read the security log, and cannot clear any of the logs
Power users cannot change the firewall settings
All of these things make rootkits virtually impossible to install as a power user. It also keeps "drive by downloads" in IE from working, and many other nasty things from happening if the user executes some bad code.
Yes, you can write to many directories and some parts of the registry that regular users can't, but to say there is little difference from a power user and an administrator is wrong.