WASHINGTON - The Obama administration wants to convey clear and concise guidance about one of the biggest national security threats in homes and offices - the computer.

The administration wants Americans to think before they click. Know who's on the other side of that instant message. What someone says or does in cyberspace stays in cyberspace - for many to see, steal and use against people and their government.

The Internet, said former national intelligence director Michael McConnell, "is the soft underbelly" of the U.S. today. Speaking recently at a new cybersecurity exhibit at the International Spy Museum in Washington, McConnell said the Internet has "introduced a level of vulnerability that is unprecedented."

The Pentagon's computer systems are probed 360 million times a day, and one prominent power company has acknowledged that its networks see up to 70,000 scans a day, according to cybersecurity expert James Lewis.

For the most part, those probes of government and critical infrastructure networks are benign. Many, said McConnell, are a nuisance and some are crimes. But the most dangerous are probes aimed at espionage or tampering with or destroying data.

The attackers could be terrorists aiming at the U.S. culture and economy, or nation-states looking to insert malicious computer code into the electrical grid that could be activated weeks or years from now.

"We are the fat kid in the race," said Lewis. "We are the biggest target, we have the most to steal, and everybody wants to get us."

Steps to improve computer security at home include:

-using antivirus software, spam filters, parental controls and firewalls.

-regularly backing up important files to external computer drives.

-thinking twice before sending information over the Internet, particularly when using wireless or unsecured public networks.