This sounds like an inspired idea. You take your TomTom GPS navigation system and, when you're on those long hauls down the motorway and you need to keep the kids quiet in the back, you convert it into a movie player and Bingo! Your kids are blissfully quiet and you can concentrate on a trouble-free journey.

X-OOM has a glut of conversion software on the market at the moment, designed to transfer films onto iPod, Wii, PSP, Pocket PC and Mobile, and obviously the temptation to add sat-navs to the collection was irresistible. However, there were ominous omens before we opened the packet, when TomTom told us they didn't support this software and consequently wouldn't supply their latest models for us to test it on.

Undeterred, a helpful soul lent us their TomTom ONE and within seconds we were all plugged into the PC and the Movies on TomTom disk smoothly and quickly performed its installation.

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Once up and running you have several choices: to transfer DVDs or selected files on your PC to your sat-nav, to your hard drive or to a CD. All common file formats such as SVCD, VCD, DivX, WMV, Real and AVI are supported and are all converted to MPEG-4 video format ready for playback. As TomTom units don't come with a built-in video player, this is installed onto your SD card together with your film.

There's an additional Expert Mode setting which allows you to build up a storyboard of movie clips of varying formats which will then be converted to the correct format for viewing. There's also a 'trim' function to make judicious cuts in each film, which can be previewed before conversion. The quality, resolution and target file size can be adjusted to fit any memory stick using the Fit-to-Stick option and the software is designed to work on Windows 2000, XP and Vista.

Sounds great? Now the downside. First of all, when you scrutinize the small print on the box cover you'll discover that encrypted or copy-protected DVDs cannot be copied to your TomTom, which pretty much dispenses with the majority of DVD feature films. So what you're left with are home movies, trailers, a few commercial DVDs and anything you might have recorded on a home DVD recorder.

Worst of all, the software itself was unstable in our tests and we failed to get it to recognise some AVI and WMV files on two separate Vista PCs. DVDs it would recognise, but clicking on destination folders usually meant the whole process froze or appeared to be waiting forever to move to the next stage. With these two major setbacks we can't see how anyone can be persuaded to use it on a regular basis.

It's hard to see how X-OOM can justify releasing a piece of software that appears to be unstable and cannot deliver anything other than home-grown films, when its publicity boldly announces that you can see “all the latest blockbuster movies” on your TomTom. Not without breaking copy-protection, you can't.