Virus Scanning from outside the OS or without running the OS is better than Scan it within the OS. Why? Cause, while you scan it within the OS that's possibility the virus still running at background and anti-virus can't delete it or clean it. So, it's better if you scan it from outside the OS that u want to scan.
As example, Virus Scanning Windows OS from LINUX OS. How? You can try to install anti-virus that can scan Windows library and also can be installed on LINUX OS, such as AVG anti-virus. You can download free version or buy the professional one from the website. The first step, You must know what kind of file system that you use for your Windows OS. If you use ntfs, you need to install ntfs-3g first. So, you can write or remove file on your ntfs partition from LINUX OS. It's needed while anti-virus clean the virus or delete the worm or trojan. The way to scan and clean the virus from Windows OS is simple.
1. Mount your Windows partition, for example to /mnt/windows/
2. run the avg from gui and scan it
or
you can run it from console by command
avgscan -clean /mnt/windows/
3. wait until the scanning process finish
4. Remember to update your anti-virus. :)
Virus that I mean is included worm and trojan horse.
I wish this will be useful for you.
Thank you...
:)
