I had to get a new hard drive in my 27 in IMac. I used time machine to get everything back on my computer. I just turn on time machine and its says it's preparing over 800,000 items to back-up, it never back-up, it stop and starts the process over again.

Could be many things, we should start with this...

"Try Disk Utility

1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.

2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)

*Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*

3. Click the First Aid tab.

4. Select your Mac OS X volume.

5. Click Repair Disk, (not Repair Permissions). Disk Utility checks and repairs the disk."

http://docs.info.apple.com/article.html?artnum=106214

Then repair the backup/TM drive.