My laptop freezes when I connect an external HDD

Whenever I connect my external hard disk to my laptop, my computer freezes as soon as I My Computer to access it or click on 'Open to view files' in the autorun menu. I cannot do once this happens. Not even task manager can be opened. The computer unfreezes and starts working fine after I unplug my hard disk. But it works just fine on my friend's laptop.
This started happening a few days ago. Till then I could use the hard disk normally without any problems. Can anyone please help me out with this problem?
My operating system is windows 7 and I am using Avast Internet Security 7 antivirus
I cannot beleive it!! after days and hours of frustration and even considering to lose all the info in my computer, this post nailed it! now I can finally use my new external HDD to back up important info and enhance my laptop performance!! THANKS!