Do you want to try Windows 7 but don't want to shell out the upgrade price? Or do you long to give Mac OS X a test run but don't particularly want to buy a Mac? There's a way to do that (although the OS vendors won't love you for it): Use a 'transformation pack'.

These software packages go beyond changing the graphical user interface to make it resemble that of a different OS. Most of the technically sophisticated transformation packs will tweak or patch the native operating system's code and add startup applications to more accurately simulate the functionality of another OS.

It's important to note that the changes these packs do to your computer's operating system are only superficial. Just because a transformation pack makes your OS look and play like Mac OS X, for instance, doesn't mean you can then install Final Cut Pro on your not-a-Mac. You may get the look, but not necessarily the full performance, and certainly not the software compatibility, of the operating system being mimicked - for that you need to run the actual OS on your system using virtualisation software such as VirtualBox.

But even virtualisation software won't let you run Mac OS X on a Windows or Linux machine, and such gaps are where transformation packs come in. They're also appealing to inveterate computer tweakers - and to anyone who doesn't want to shell out for a second OS just to give it a quick spin.

Be warned, however: These OS makeovers are all unauthorised. Apple in no way endorses any of the transformation packs that turn Windows or Linux into an ersatz OS X, nor does Microsoft support those giving Windows XP a facelift to make it look like Windows 7.

These packs are basically works of fan art, and their creators give them away for free. Most directly appropriate (translation: rip off) the icons, wallpaper and other copyrighted art of the actual operating system that they turn your computer's original OS into, which means that by using them, you could be held liable for violation of copyright law. Use them at your own risk. (It goes without saying that you shouldn't install one on a computer you don't own.)

I took a look at four of these transformation packs. This is by no means a comprehensive list, but rather an overview of some of the most popular. I tested how easy each was to install (and to uninstall), how useful it is in practical terms (i.e., would you really want to use it for your day-to-day computing?) and, very importantly, whether the pack can adversely affect your computer's original OS. (Remember, many of these packages will alter system files and add programs, which can bog down your computer's performance or even lock up your operating system. Always back everything up before installing one.)

I ran all four packages on a very basic 2GHz Pentium Mobile notebook with 2GB of RAM and no built-in 3D graphics. It dual-boots Windows XP Home and Ubuntu Linux. Even on this unimpressive machine, most of the transformation packs ran reasonably well. Keep reading for my assessments of each one.