You know,like a floppy drive,can a harf disk be write protected?Coz I need to use my hard disk at a friend's place who's PC ios severely infected,I hope to use my antivirus to c;eanse his system as no AV installs onthat system.

In reality you can't or you shouldn't because then you wouldn't be able use the hard drive, this link should provide a good answer, and maybe this link as well. However what I suggest is that you bet a whole bunch of software that consist of a registry cleaner (updated), Spybot (fuller updated), Anti Virus software (fully updated), Firewall (updated), and install the latest windows updates. Now after doing all that disconnect the computer from the internet and then go through the process of cleaning out the computer and then make sure you have the windows firewall on and the anti-virus have that firewall on as well.

Or if your friend's computer has nothing important on it, reinstall the operating system and then install all that protection, either eay I high discourage putting your hard drive into that computer because you could suffer the same fate as him.

Look, the hard disk is a device to read and write information -all the information, without discriminate something.

All the media has that function, to store data. The only way to avoid to infect a hard disk (or floppy, flash drive, etc.) is just to keep unplugged and without store nothing on it.

As Saint_Michael said, the only way to stop some of the viruses, is that, using any kind of AV, Antispyware and Optimizers to keep the hard disk without the most common infections.

But, the real thing, is that the only way to keep your hard disk without problems is, at first place, avoid sites that contains dangerous active-x and javascripts, sites like warez and porno.

More restricted options are, limit the number of users and talk to them to take practices that avoid to get contaminated or, also, missconfigure the whole system.

What you could do is download a LiveCD Linux Distro and try to disinfect your friend's HDD. You could install WINE and run the anti-virus program through that, i suppose. That way your HDD won't be proned to be infected with the same virus. I do not guaruntee that this will actually work, but it's a start. Besides that, you can reformat the HDD and re-install the OS or install a different OS. Though, if it's a smart virus, reformatting and re-installing the same OS may not help.