The OP already said he can boot into Safe Mode, so why are people saying boot into safe mode?
To the OP - Since you can boot into Safe Mode fine, most likely there is a driver or another piece of software that is causing the issue. It could also be a virus/spyware/malware that is interfering. Booting into Safe Mode boots the computer with a minimum of drivers and no start-up programs. It allows you to diagnose software issues that can be causing problems.
Once you are in safe mode, run MSCONFIG - hit the start button and type msconfig in the search box. If you turn off (un check) everything in Services and Start-up, you can turn them on one by one until you encounter the problem. Whichever program or service you just checked is the one causing the problem. Unfortunately, if the problem is with a driver, MSCONFIG will not help.
I too question a failing hard drive. If the drive were failing, you most likely could not boot into safe mode. I would check the Windows System Log in the Event Viewer (Control Panel > Administrative Tools > Event Viewer) and see if it reports an error for the hard drive as well.
It seems like a waste of a drive bay (especially in a laptop) to have an internal backup hard drive. I would find out for sure if you have a RAID 1 - two hard drives that are mirrored into one, it works sort of like a backup so that if one drive fails the other will allow the computer to continue to function until you can replace the hard drive. If you have a RAID 1, you can replace the failing drive and rebuild the RAID from the functioning drive. If it really is just an extra, back-up drive you'll need a new hard drive to restore the back-up onto.
-Michael