Cannot log into ESX 3.0 host via putty, winscp, the mui or ssh as root ?

More than likely because ESX is secure by default and does not allow it.
To change it modify the /etc/ssh/sshd_config file and change "PermitRootLogin no" to "PermitRootLogin yes"
You will need to do this via VI on the console. Follow the steps below.

Login to the console as root

• [root@<machine name> root]# cd .. (enter)
• [root@<machine name> /]# (note the prompt has changed, the root has gone)
• [root@<machine name> /]# cd etc
•   [root@<machine name> etc]# cd ssh
• [root@<machine name> ssh]# vi sshd_config
• This will open up the editor vi and you will be looking at a file. 
• Scroll down until you find PermitRootLogin yes
• Scroll across until you are the character before the word "no"
• press i , then then delete the word "no" and replace with yes
• Press ESC key twice
• Type        :wq!       (including the : )   Then ENTER
You should be back at the prompt [root@<machine name ssh>#
Then type service sshd restart