You can get a free ssh client (PuTTY) at:
http://www.chiark.greenend.org.uk/~sgtatham/putty/
One you have it installed:
1. Open PuTTY.
2. Connect to your server:
# ssh username@IPAddressHere
Log in as your Primary User, enter your password when prompted for a password.
3. Once you are logged in, you should see this:
[username@hostname username] $
4. Su to root:
# su -
Type in the root password when prompted for a password.
5. If you su'ed successfully, your prompt should look like this now:
[root@hostname root] #