Well, the RUU relies on a program (reboot oem-78) in the main OS to boot the phone into a special "RUU" mode - and your new ROM does not have that same "reboot" command available.
Here is what you can do.
1) Run the RUU program right up to the spot where you would normally try to start programming the phone. Don't do anything though - just leave the RUU utility sitting there (do not close it).
2) Open up Windows Explorer, and go searching inside
C:\Documents and Settings\<Your_User_Name>
for a big file named "rom.zip" - probably around 100+ MB
You might need to configure Windows Explorer so that it will display "Hidden Files and Folders" - it won't search in areas that it won't display (Tools-> Folder Options->View in Xp, Organize->Folder and search options->View in Win7)
3) Copy that file to some other folder outside of C:\Documents and Settings, and rename it to PB00IMG.ZIP. Note that (in Windows Explorer) you may also want to temporarily toggle off the folder option "Hide extensions for known file types" so that you don't accidentally name it PB00IMG.ZIP.ZIP
4) Now Copy this file to the root folder of the SD card of your phone. If you do this using the USB cable, please please please "unmount" the drive from Windows the correct way: (a) Use the "Safely Remove Hardware" control in your system tray to disconnect the device. You should see a balloon pop-up telling you you "safe to remove hardware" - if you see a message telling you that you still have files open on the device, you need to click on a folder in your C: drive somewhere in Windows Explorer, and then re-try the "Safely Remove Hardware" control. Then (b) turn off "Disk Drive Mode" on the Eris
5) Make sure your phone is fully charged, and shut off your phone.
6) Turn your phone on by holding down the Vol-Down & End/Power buttons
If everything went well, this will start "HBOOT", and it will find the PB00IMG.ZIP file automatically, and scan it to make sure the file integrity is OK. If it gets this far, it will ask you to proceed. Press the right button to make sure that this happens.
If you want to look at some pictures of what this process looks like, you can have a gander at
this post.
Do not interrupt the phone for several minutes. It will take up to 10 minutes for it to completely reboot and be back with a fresh copy of 2.1. It may also automatically discover the July 2010 OTA update and ask you to install that.
Good Luck.
eu1
PS If you made a Nandroid backup of your Factory ROM before you installed the custom ROM, you could just restore it using the Amon_RA custom recovery boot. That would be a lot easier than the above procedure, and you would end up right back with what you started with, fully configured.
That's what Nandroid backups are for.
