How can I restore the EFI? partition

I'm not sure where you got that information but I think you either misunderstood or were mislead. The prohibitory icon doesn't mean what you think:

http://docs.info.apple.com/article.html?artnum=106805.

Boot.efi is a file on the hard drive. If is one of many system files. Per the document reference I sent to you if any of these files are damaged, missing, or corrupted they will cause the same warning.

"Circle with Slash - could not load boot.efi, or some other issue."

This has nothing whatsoever to do with partitions on the disk drive. You just need to reinstall OS X. In fact you probably can do so using Archive and Install and not even have to erase the hard drive:

How to Perform an Archive and Install

1. Be sure to use Disk Utility first to repair the disk before performing the Archive and Install.

Repairing the Hard Drive and Permissions

Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Installer menu (Utilities menu for Tiger.) After ** loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the ** status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If ** reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported, then quit ** and return to the installer.

If ** reports errors it cannot fix, then you will need Disk Warrior (4.0 for Tiger) and/or TechTool Pro (4.5.2 for Tiger) to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

2. Do not proceed with an Archive and Install if ** reports errors it cannot fix. In that case use Disk Warrior and/or TechTool Pro to repair the hard drive. If neither can repair the drive, then you will have to erase the drive and reinstall from scratch.

3. Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When you reach the screen to select a destination drive click once on the destination drive then click on the Option button. Select the Archive and Install option. You have an option to preserve users and network preferences. Only select this option if you are sure you have no corrupted files in your user accounts. Otherwise leave this option unchecked. Click on the OK button and continue with the OS X Installation.

4. Upon completion of the Archive and Install you will have a Previous System Folder in the root directory. You should retain the PSF until you are sure you do not need to manually transfer any items from the PSF to your newly installed system.

5. After moving any items you want to keep from the PSF you should delete it. You can back it up if you prefer, but you must delete it from the hard drive.

6. You can now download a Combo Updater directly from Apple's download site to update your new system to the desired version as well as install any security or other updates. You can also do this using Software Update.

Unfortunately the only option is to re-partition the hard drive then use Boot Camp to again create the Windows partition.

When the Windows installer runs it will need to format the Windows partition on the drive. However, the formatter will see all the partitions on the drive. You must be sure to select the C: partition created by Boot Camp to avoid having the Windows formatter attempt to format the wrong partition on the drive.

The Firmware Restoration CD restores the computer's firmware. It does nothing to the hard drive itself.

As for Disk Utility it will only show the user partitions on the drive. Any other partitions created are invisible to the user. I'm not sure why you think there's is an actual problem other than the user partitions have been messed up. Why don't you first try using Disk Utility to erase the OS X partition then reinstall OS X. You may find that will restore OS X. You can then attempt to reinstall Windows being sure you choose the proper partition for the installer to format.

Well, until you actually try erasing the OS X volume and reinstalling OS X, you can't be sure there really is a problem. Had the Windows formatter actually overwritten the GUID partition block there would be pretty good evidence of it. Among other things your OS X volume would no longer show as Mac OS Extended nor would the drive's partition scheme still appear as GUID in the Disk Utility status area.

If the OS X volume is still accessible you should be able to access it in Target Disk Mode from another Mac. I think you should first assume that the drive is intact and erasing the OS X volume and reinstalling OS X is all you need to do. You can try TDM to recover your files beforehand in case they are accessible. If all this fails, then you will need to start over by re-partitioning the drive.

In the ** status area if you select the hard drive entry (mfgr.'s ID and drive size) you should see a line under the SMART Status labeled "Partition Scheme." It should say GUID Partition Table. If it does there's nothing wrong with the drive partitions.

BTW, there is no "EFI" partition. The "boot block" partitions on the hard drive are not even visible in Disk Utility. You would need to use one of the Unix tools to actually see that part of the drive structure.

Actually, you can boot an Intel Mac from a drive partitioned GUID or APM. You just can't install the Intel version of OS X on an APM volume or a PPC version on a GUID volume using the Apple installer - it won't permit it. But you can clone an installed version of OS X (Intel) to an APM volume and it will boot an Intel Mac just fine.