Worried about people accessing your private information whenever you use a public computer?

There is a way to protect yourself: Devices as small as a keychain allow you to use any computer without leaving a trail of evidence.

A new computer program known as MojoPac can turn most flash memory sticks, hard drives or iPods into virtual PCs that can run most programs that work on Windows XP.

The devices draw on the host computer's resources -- including its electricity, Windows XP software and DVD drive. Yet they retain their independence as they move from machine to machine.

This independence allows people to use public computers without a trace of their session being left behind. PCs typically store a record of activity long after the computer has been turned off. It's a slick way to move from machine to machine, says Rob Enderle, founder of the Enderle Group, a research firm that follows the PC industry. It's about as safe as you can get.

The device cannot be bought. You have to make it by downloading free software onto a computer drive such as the thumb-sized USB flash memory drives that were so popular as gifts this Christmas. It also works with iPods, many other digital music players and regular external hard drives.

Once the MojoPac shell is created, users need to install their own software -- just as they would do on a regular PC running Windows XP.

MojoPac's developer, RingCube Technologies, asserts that most programs are compatible, including Microsoft Office, Adobe Photoshop and a slew of free programs.

Other companies are working on similar technology, but there's nothing available with free software that is as easy to set up as MojoPac, according to Enderle.

MojoPac virtual PCs are not just designed for mobile use. They can protect users who share the same computer. A virus introduced by one user into their MojoPac, or virtual computer, would not affect the rest, according to RingCube.

If you were to corrupt your virtual world, your host PC would be fine, says RingCube Senior Vice President Ron DiBiase.

MojoPac is available for free on the company's Web site, http://www.MojoPac.com.

For now it only works with Windows XP, but the company plans to launch a version this summer that allows users to switch between machines running XP and Vista.

PCs that have been locked by administrators so users cannot install files on them won't work with MojoPac unless the administrator first installs a small piece of software that is available on the company's Web site.

And, there are no plans to develop MojoPacs for Mac computers or the Linux operating system, DiBiase said.

While most programs work with MojoPac, one good source for the devices is http://www.PortableApps.com, a site that specializes in offering programs customized for thumb drives.

It lists more than three dozen programs, including software for fighting viruses, backing up data, surfing the Web and viewing documents. There are also programs for word processing, photo editing, spreadsheets and instant messaging.