This is no longer the case. Viruses are now more dangerous and spread much more easily. Some CMOS setups can be configured to prevent writing to the boot sector of the hard drive. This may be of some use against boot sector viruses. However, if you need to reinstall or upgrade the operating system, you will have to change the setting back to make the MBR writable again.
Most antivirus packages contain routines that can perform both tasks.
A good virus protection software package is Norton AntiVirus . Back up your files, so that you can restore them if a virus damages them. Check your backup files with virus scanning software before using them. This will prevent the virus from spreading to your original disks. If you must insert one of your application floppy disks into an unknown computer, lock it first.