OK Tex, here goes:

i found out that there computer system runs windows 98 ( not secure!) on a pc out in the open for anyone to access if they want.
Win98 is not a secure operating system as such, HOWEVER no OS is secure if you allow unrestricted physical access.

"Out in the open" might in fact be a defence in itself, as anyone using it can be seen by everyone else?

The real issues are:
1. What is it being used for?
2. What data does it contain (security/confidentiality)
3. Does it connect to the internet? If so, how?
4. How business critical is it?

Simply changing the operating system will do very little for you. You need to address the other aspects such as:

1. Authorised usage policy
2. Firewall
3. Antivirus
4. IDS
5. Anti Trojan
6. Anti Spyware/Adware
7. Backups

I strikes me as rather strange that in an affluent and technologically aware part of the World, someone would be using a Win98 box for anything but the most trivial activities?

The way to "sell" in your environment is going to be legal compliance (confidential data), business criticality (how long can you live without it) and the importance of the data being stored on it.

You really need to find out more