What Operating system are you running?
If you suspect a hard drive failure, you can test it with a program called UBCD (Ultimate Boot CD). This program has hard drive testing tools that you can run and will give you a pass or fail result. If your hard drive has bad sectors, or is failing in anyway this CD will let you know. Here is the download link: http://www.ultimatebootcd.com/download.html
If your hard drive tests good... the next step is to see if you can boot the computer into safe mode successfully, which can be done by tapping the f8 key continuously as the computer is booting and then selecting "Safe Mode" from the menu.
Let us know your results