why some of the .exe softwares can't be installed on my pc?

More details would be helpful. - Are you logged in as administrator when you try to install them? - What software were you trying to install? - What error messages do you see when you try to install them? Example: do they say that certain files must be installed for the program to work?

To give a good answer, you need to give me first some more information mate, because I can't do anything with this. One thing I can tell you for sure. If you have antivirus software isntalled, and what you downloaded is a virus, spyware, or MAY contain spyware, then it won't allow you open it. That could be a reason..but for the rest, you'd need me to give more details mate! And check what kind of Windows XP you've got..which service pack..you can see it when you go to start configuration system and then you'll see it soon enough..I can give some more possibilities why, the moment you've passed on more details!