If you've been using a computer for some time and need to abandon it -- either because you're leaving a job or moving to another machine -- you need to be concerned about security. Once you leave, a lot of information can be retrieved about you just by inspecting the digital traces you leave behind.

Clear History, for instance, removes the history of activity kept by most popular applications. It can clear not only document and Internet history but also can ferret down into more arcane data and registry information, removing any record of your activity.

So before you say goodbye to a PC, follow this list of to-do items to ensure that no one gains information about you that they do not need to know.

Apps tell a lotThe applications on your PC keep a record of almost everything you do. By default, your Web browser probably leaves tracks from a lot of sites you have visited, stores the user names you use online, files you have downloaded, and even passwords, if you allow it.

Many business applications also keep track of the documents you have last worked on or edited. "Histories" are useful to those who need quick recall of what they last worked on, but they are anathema to those who require privacy.

There are ways to selectively clear usage history with each of the applications that keep such records. But because it's difficult to remember exactly which applications are keeping a record of how you have used a computer, a better solution is to turn to a tool that specialises in removing traces of every document you have worked on or website you have visited.

Don't trust your Recycle Bin

If the PC you have been using contains sensitive files, it is not enough simply to delete them. Even if you delete a file and subsequently remove it from the recycle bin, a savvy user can reconstruct deleted data and potentially gain access to files that you thought were gone for good.

So to delete sensitive files, you have to turn to something more robust than the delete key. Software-based file shredders fit the bill. Most of these applications work by scrambling the contents of a file using special algorithms.

Remember, mail is 'official'

Your email probably holds a plenty of information you would rather not see fall into the hands of someone who comes along to use a computer after you. So it's critical that you know how to destroy email you do not want others to see.

First, though, deleting email from a machine that has been connected to a corporate network probably does not actually remove all traces of messages you have received or sent. Many companies are required by law to keep copies of the email their employees generate, so your best course of action is to plan ahead and never send sensitive information by email from work.

You can delete email messages individually using the Delete key on particular messages. Remember, though, that deleting email messages this way just sends them to the Deleted E-mail folder, where they will need to be deleted again. Holding down the Shift key, while deleting messages sends them directly to the recycle bin in Outlook and Outlook Express.

Formatting is not the answer

If you have the option to completely wipe out a hard drive before leaving a machine or returning it, take advantage of it. A study by researchers at Britain's Glamorgan University showed that more than half of the used hard drives purchased from eBay contained retrievable personal and financial information.

Reformatting an old hard drive isn't enough, since even a standard reformat can leave the data on a disk vulnerable to retrieval by savvy users.