Apple hardware test can easily miss problems or detect false ones. Run the test in 'normal' & 'extended' modes on a loop for several hours (or overnight) to be sure that the hardware is OK. Press cmd+L to enable loop mode in the hardware test.
In addition to lllaass's suggestions, try booting from any bootable backups if you have them. Try booting from an OS X install disk too. If these fail you may have a hardware fault.
Try safe mode to disable third party software extensions & startup jobs…
Try safe mode if your Mac doesn't finish starting up - Apple Support
If it works like that post back, it is normal for a safe boot to feel 'laggy'…
Limited graphics performance in OS X recovery or safe mode - Apple Support
You can also use verbose mode to view some of the startup text, but you may have to record it with a camera to be able to see the errors. It can scroll by quickly.
Mac OS X: How to start up in single-user or verbose mode - Apple Support
Also take a look at this for reference, it may help you work out what parts complete before it fails.
About the screens you see when your Mac starts up - Apple Support
One more thing to consider. OSX can be reinstalled without erasing the disk. Download the latest 10.7 installer & write it to a USB stick (http://diskmakerx.com/), then boot that use it to 'reinstall OSX' it may fix issues with damaged system files. Just have a full backup before you begin - it could overwrite files if you choose the wrong options.
Recovery mode can be used to backup a working disk too, so ask if you need to do that.
