Major Boot problem!

Question

All of a sudden my iMac is taking 45 mins + to boot up!!
I've been reading through the forums trying to find out what the problem might be and resolve it.
It normally gets to the Grey screen with the apple logo and spinning gear then freezes it does eventually boot but after quite a long time.
As I understand it by how far it gets in the boot sequence this might be related to the kernal, but thats where my knowledge end with regards to the kernal.
Heres what I've tried so far:
- Boot from system disk & ran verify & repair on HD and any external drives
- Reset PRAM
- Unplugged all peripherals
- uninstalled recently add 3rd party software
Is there any thing else I might be able to try or do I need to just do a complete clean install?
Any guidance is very much appreciated
I'm running iMac 21.5" Late 2009
1TB HD
8GB DDR3
ATI Radeon HD 4670 256 MB
OSX 10.8.4 (12E55)
Thanks

Answer 1

Back up all data immediately, as your boot drive may be failing.
If you have more than one user account, these instructions must be carried out as an administrator.
Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
Step 1
Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select
View ▹ Show Log List
from the menu bar.
Enter "BOOT_TIME" (without the quotes) in the search box. Note the timestamps of those log messages, which refer to the times when the system was booted. Now clear the search box and scroll back in the log to the last boot time when you had the problem. Select the messages logged after the boot, during the time something abnormal was happening. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V).
For example, if the problem is a slow startup taking three minutes, post the messages timestamped within three minutes after the boot time, not before. Please include the BOOT_TIME message at the beginning of the log extract.
If there are runs of repeated messages, post only one example of each. Don’t post many repetitions of the same message.
When posting a log extract, be selective. In most cases, a few dozen lines are more than enough.
Please do not indiscriminately dump thousands of lines from the log into this discussion.
Important: Some private information, such as your name, may appear in the log. Anonymize before posting.
Step 2
Still in Console, look under System Diagnostic Reports for crash or panic logs, and post the entire contents of the most recent one, if any. In the interest of privacy, I suggest you edit out the “Anonymous UUID,” a long string of letters, numbers, and dashes in the header of the report, if present (it may not be.) Please don’t post shutdownStall, spin, or hang logs — they're very long and not helpful.