For the past 25 years I've been in pursuit of an adventure that started out being called ‘the micro' and evolved into ‘the personal computer'. It's a playground of pure logic, and a bunch of very clever people – program designers and programmers – have been having an insanely great time discovering that they can do pretty much anything and everything with it. They have been only too happy to share the results with the rest of us – some sharing for money, but many just for fun and fame. The bottom line is that this multifaceted, shape-shifting thing we call the PC managed to bring an entirely new dimension into all our lives.

Then comes the Internet and the opportunities go exponential. The entire, wonderful, frame-breaking, history-rattling, epoch-exploding venture turns from an ingenious caterpillar into a soaring butterfly. And then the butterfly congeals into, a shopping mall?

I want to play my collection of music, so I power up iTunes and lo, I'm looking at a shop. A quest for information? I only have to bring up Google – but now riddled with paid-for search responses and links to its own multifarious commercial ventures, Google has also become a shop. Even applications I've known and trusted for years now pop up unwanted windows inviting me to buy ‘Power Upgrades'. It stinks, but it could be worse.

You can bet it will be. What's happening with these pop-up windows and these unsolicited adverts is that your personal computer is turning into their corporate billboard. But this hijacking of your screen is only the appetiser. Corporations are now assuming that what goes on inside your PC is also their rightful turf. Big business is set to take over a slice of your hard drive and a regular percentage of your processing power.

Last year Sony's notorious rootkit exploit in the name of ‘copy protection' was the first widely publicised hint that global corporations (other than Microsoft, of course) saw your computer as theirs by right. Towards the end of last year, something similar happened when I became one of the 5,000 beta testers on the BBC's IMP Trial, a scheme for downloading encrypted, digital rights-managed TV and radio programmes. These downloads didn't come direct from BBC's own server; instead they used a proprietary form of BitTorrent filesharing called Kontiki (www.kontiki.com). BitTorrent technology uses the processing power of each client computer to share the workload: clients are uploading as well as downloading. I've been tracking BitTorrent since it started in 2001, but the open source variants I've used (Azureus, uTorrent and others) deliver precise reports on how the software is using your computer and Internet connection. Kontiki does none of this. Mention of its existence is buried in the EULA for the IMP Trial software, but beyond that Kontiki's strategy is to duck out of sight and take care of business without bothering the user's pretty little head.

BBC's IMP trial closed at the end of February, but Kontiki is also the download motor hidden inside Sky's equivalent ‘Broadband on Sky' software. Users don't notice Kontiki as such, but they do discover a tendency for their PC and Internet connection to go into slow motion once the software's been installed. A key problem is that most broadband connections are asymmetric, with an upstream channel that is much slower than the downstream channel, and so easily clogged by files being sucked back by that untold number of Kontiki clients on the Internet. Kontiki offers the user no fine tuning, the only way to control it is to shut down the PC or break the Internet connection. But my main objection to Kontiki is that it's a kind of legitimised (because you agreed to the EULA) zombie trojan that 0wnz Ur PC.

You don't even have a sense of ownership of the files Kontiki delivers to you. The BBC IMP trial has been my first prolonged exposure to DRM content, and it's not an experience I'm eager to repeat. Yes, you can download a copy 'Hancock's Half Hour' that BBC 7 transmitted a week ago, but it's encrypted and requires an Internet-delivered licence before it will play. This licence is time-limited, so you had better listen to it right away. And it will only play on the machine you downloaded it onto, or on an officially authenticated portable device that's bought into Microsoft's DRM scheme.

Once we've become acclimatised to this, the next step will be 'Trusted Computing', where the PC you've bought and paid for is hardwired to become the entire property of the corporate entities who deign to supply you with applications and content. At this point, no longer ‘personal' and hardly any longer a computer, the PC adventure will definitively be over.