Hello,
my external hardisk is not accessible, when i click that folder one message box appear & some message written there such as - "H\SOFTWARE(a folder) is not accessible. plz help.

Most probably it is infected but before that you should also check if hard disk is in good state. For that when hard disk connected to computer, right click on it and under tools, check it for errors. If it shows no error then all you need is to scan your hard drive with antivirus software given that your computer is also not infected.

Btw, which external hard drive it is and how old ?