My Hard Disk is not accessible from "my computer"?

I can't access the contents of my hard disk from "my computer". Each time I double-click, a new window says I have to choose the program I want to use to open the file. However, I can still view the contents of my hard disk by using the "run" facility and typing "C:\" in it. I have tried to restore my computer to an earlier state but the problem persists. I don't have a virus, I have checked twice with my up to date antivirus. I have the impression that the problem occured after the automatic windows update and installation. When I bought my pc 4 years ago, the Windows XP home edition installation CD was not provided. Therefore I am not able to reinstall the Windows. I think the problem is located in Windows Registry. I would really appreciate if anyone could give me a good solution to this ennoying problem. Thanking you in anticipation.