Are the viruses too old to worry about?
No, they can still be a problem. Except, I guess, in the incredibly unlikely event they use old 16-bit code and you are using an edition of Windows with no 16-bit compatibility, they can still technically run, and virus code is often simple enough that it doesn't have too many dependencies on specific libraries so there is a decent chance it'll run on a future version of Windows.
That said, there are some tricks in modern Windows that can make them less likely to be effective:
- Firstly, Windows now comes with a virus scanner and malware detector, so it's more likely to be detected.
- Windows' user account control means that most software is now run with limited ability to modify the system unless you pass a prompt.
- Viruses may sometimes target vulnerabilities in software that has since been patched.
But all this doesn't mean you can ignore them or that they can't cause harm - you still should try to make sure you have no viruses on disk, whether dormant or executing, whether they still actually work or not, to be safe.
Should I format my PC?
The problem with formatting everything and starting again is that presumably there is data on that drive you want to keep. This is a problem with old hard drives and even media like CD-ROMs with backup data on them - they can contain old viruses too.
A reasonable strategy would be to try and disinfect the drive using the methods available in your antivirus suite. Make sure all your software (including things like Microsoft Office) are up to date including security patches, and you don't accept any security prompts you don't understand when opening documents.