Even before a person connects their computer to the internet they should have an excellent anti-virus software. Most of the good software out there today has a remote server that can automatically update your virus protection.
You can also install a scanner that stays in your memory and scans all files before you open them, this is especially handy when using floppy disks.
To fully fight viruses you must know how they work. How they infect other files. Viruses can be made by overwritting the beginning of the file replacing it with their own infected code; usually these viruses are easy to detect because the program itself won't work, just the virus will. They also can be made adding a jump to the end of the file, where its code is located, and then continuing with a jump back to the beginning where it left off; these are usually harder to detect except they use a little bit more memory. The last type is the virus attached to the beginning of the file, but it doesn't change the makeup of the original. This the hardest to detect but this depends on how well it is written.