Statistics collected from exactly 20,000 users over a seven day period show that only 1.91 percent of PCs have exactly zero insecure programs.

OK, so the 98 percent headline number is an overstatement in that the study only looks at Windows PCs, not those running Mac OS X or Linux. We'll overlook that for now. 98 percent of 90 percent (or whatever percentage of the installed base is running Windows) is still a shipload of computers.

Denmark-based vulnerability intelligence provider Secunia offers a free tool called Personal Software Inspector (PSI) that's used to check PCs for software that hasn't had the latest security patches applied.

We're quite prepared to believe Secunia's IT development manager Jakob Balle when he says the figures collected by the company are likely to be "best case" because "The users of the Secunia PSI are likely to be more vigilant and security minded than all other Internet users."

If you weren't worried about keeping your software up to date, you probably wouldn't use PSI. On the other hand, if you were extremely confident your installation was up to date, you probably wouldn't bother with Secunia's utility.

On the other hand, PSI is only free to home/private users, so the results are presumably skewed away from professionally-managed systems that you might expect to be kept more up to date.

But is that necessarily the case? Please read on.

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However, many IT operations seem to favour stable system images, and therefore won't deploy patches without extensive testing. Some experts think this is wise, others believe the risk of attack is too high for it to be a viable strategy.

On balance, we'll go with Balle's interpretation.

But what's the breakdown of the 98.09 percent of computers that aren't fully patched?

30.27 percent had between one and five insecure programs, 25.07 percent had six to ten, and 45.76 percent had eleven or more.

Yes, almost half of the PCs running PSI had more than ten unpatched programs!

On reflection, that's perhaps not as surprising as it seems. A careful user may have a handful of unpatched items at any time, despite most of the software being up to date. Even where applications automatically check for updates, there may be a delay before they are patched. If a program isn't running, then any vulnerabilities remain theoretical.

But careless users probably don't apply patches that aren't automatically presented to them, whether that's through Windows Update or other vendors' auto-update services.

So the idea of nearly half of all Windows PCs having more than 10 vulnerable programs doesn't seem so outlandish after all.

If you want to try PSI, you'll find it here.