My computer was stolen. It's password protected, what files can the thieves see?

There's a fundamental premise that I try to remind people of from time to time. It's simply this:

If it's not physically secure, it's not secure.

Now, normally I bring that up when people have questions relating to sharing a computer, or perhaps sharing living space and being somewhat concerned about what a roommate might or might not have access to when the computer's owner isn't around.

The short version is that if someone has physical access to your computer, they can quickly gain access to everything on it.

And of course computer theft is the very definition of physical access.

There are a couple of ways that - with physical access - someone can gain access to your computer's contents:

• They can reboot from a CD and reset the administrative any login password. In fact, it's so easy here are the instructions: I've lost the password to my Windows Administrator account, how do I get it back?

• They can reboot from a Linux live CD and access the contents of your hard drive without needing to log in to Windows at all.

• They can remove the hard disk from your machine, install it into another, and once again access the contents of your hard disk without needing to login to the Windows that you had installed.

All that should be pretty scary, mostly because it is.

So, again:

If it's not physically secure, it's not secure.

So what do you do?

Well, in an after-the-fact case like you're asking about, it's too late. The computer has already been stolen. What's important now is that you know that the data on it could be accessed by whoever has the machine now. If you have personal and confidential information on it, it's time to assume that it's been completely compromised. It may not be. It may not be yet. It may never be. But you must simply assume the worst.

There are two approaches to prevention:

• Physically secure the machine.

• Secure your sensitive data.

Securing your machine means doing things like bolting or cabling it down or putting it in a locked room or cabinet (watch the ventilation, if you do the cabinet!). These aren't perfect solutions, as a determined thief might well still be able to circumvent these measures, but they'll at least thwart the casual burglar by making it easier to steal something else.

The real solution, in my opinion is to secure your sensitive data using encryption. In fact, it's exactly like the steps you would take to keep data on a laptop secure. By definition, a laptop is portable and very easy to steal. Hence, the solutions that keep data on laptops secure will work just as well for your desktop computer.

There are many solutions for encrypting your data. I personally don't recommend using Windows own native encrypted file system, simply because it's too easy to lose access to the data yourself if you lose access to the Windows login account that created it. Much easier is TrueCrypt, which is free, open source and cross platform. You protect your data with a passphrase, and the encrypted container can be easily copied to any machine, even non-PCs, and accessed with that passphrase.

It takes a little work to set up, but I heartily recommend using a solution like TrueCrypt at least for your sensitive data. (You can, if you're so inclined, encrypt your entire hard drive using TrueCrypt, but I personally find that's overkill for most folks.)

Once encrypted with a strong passphrase, if your computer does get stolen, the thieves can access only the meaningless encrypted data, and not your sensitive files.