It would be helpful for you to be more descriptive about the current state your phone is in. That will give us a starting point.
(1) First, what bootloader is present?
Power off (if possible, or do a battery pull) and remove the SD card.
Turn the phone on by holding down Vol-down and End/Power (red button).
Report back here the first six
lines of the display.
Optionally, if you know how to do it, report the md5sum of the PB00IMG.ZIP file you were using to attempt root with.
Edit: If you have not already done so, download the correct fastboot executable
for your computer. (Note that there is nothing else on that page of relevance to you except for the fastboot program - all the other images listed are not for the Eris). While we're at it, what kind of computer are you connecting your phone to: Windows (which version?), Mac OS/X, or Linux?