iMac 27 OS 10.6.8 Stuck on Apple logo when trying to re-install

Have you reinstalled Snow Leopard? Before doing that see the following:

Kappy's Personal Suggestions for OS X Maintenance

For disk repairs use Disk Utility. For situations DU cannot handle the best third-party utilities are: Disk Warrior; DW only fixes problems with the disk directory, but most disk problems are caused by directory corruption; Disk Warrior 4.x is now Intel Mac compatible. TechTool Pro provides additional repair options including file repair and recovery, system diagnostics, and disk defragmentation. TechTool Pro 4.5.1 or higher are Intel Mac compatible; Drive Genius is similar to TechTool Pro in terms of the various repair services provided. Versions 1.5.1 or later are Intel Mac compatible.

OS X performs certain maintenance functions that are scheduled to occur on a daily, weekly, or monthly period. The maintenance scripts run in the early AM only if the computer is turned on 24/7 (no sleep.) If this isn't the case, then an excellent solution is to download and install a shareware utility such as Macaroni, JAW PseudoAnacron, or Anacron that will automate the maintenance activity regardless of whether the computer is turned off or asleep. Dependence upon third-party utilities to run the periodic maintenance scripts had been significantly reduced in Tiger and Leopard. These utilities have limited or no functionality with Snow Leopard or Lion and should not be installed.

OS X automatically defragments files less than 20 MBs in size, so unless you have a disk full of very large files there's little need for defragmenting the hard drive. As for virus protection there are few if any such animals affecting OS X. You can protect the computer easily using the freeware Open Source virus protection software ClamXAV. Personally I would avoid most commercial anti-virus software because of their potential for causing problems.

I would also recommend downloading the shareware utility TinkerTool System that you can use for periodic maintenance such as removing old log files and archives, clearing caches, etc. Other utilities are also available such as Onyx, Lion Cache Cleaner, CockTail, for example.

For emergency repairs install the freeware utility Applejack. If you cannot start up in OS X, you may be able to start in single-user mode from which you can run Applejack to do a whole set of repair and maintenance routines from the commandline. Note that AppleJack 1.5 is required for Leopard. AppleJack 1.6 is compatible with Snow Leopard. There is no confirmation that this version also works with Lion.

When you install any new system software or updates be sure to repair the hard drive and permissions beforehand. I also recommend booting into safe mode before doing system software updates.

Get an external Firewire drive at least equal in size to the internal hard drive and make (and maintain) a bootable clone/backup. You can make a bootable clone using the Restore option of Disk Utility. You can also make and maintain clones with good backup software. My personal recommendations are (order is not significant):

Backuplist

Carbon Copy Cloner

Data Backup

Deja Vu

iBackup

JaBack

Silver Keeper

MimMac

Retrospect

Super Flexible File Synchronizer

SuperDuper!

Synchronize Pro! X

SyncTwoFolders

Synk Pro

Synk Standard

Tri-Backup

Visit The XLab FAQs and read the FAQs on maintenance, optimization, virus protection, and backup and restore.

Additional suggestions will be found in Mac Maintenance Quick Assist.

Referenced software can be found at CNet Downloads or MacUpdate.

As a start I would suggest using a utility like Snow Leopard (now Lion) Cache Cleaner or a similar utility such as TinkerTool System to clean all user, system, and font caches. Also:

Open Activity Monitor in the Utilities folder. Select All Processes from the Processes dropdown menu. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time, then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.

And, visit The XLab FAQs and read the FAQ on resolving SBBOD issues.

Yours may simply be an issue of cleaning up. You can try creating a new user account. Log into the new account. Do all these problems disappear?

The repetitive messages from Disk Utility are normal and may be safely ignored. I would be a bit concerned by the fact that you cannot successfully boot from an installer disc.