Nope, Dan is spot on.
Essentially it sounds like your friend has used a copy of someone else's Vista operating system, so strictly speaking you are in breahc of copyright, as you have not purchased a copy of Vista but are using a pirated version. Short of buying a new version of Vista and having it installed, you won't get rid of that message.
Microsoft are gradually catching up on dodgy copies of installed operating systems and software - going back some months, one of the security updates they put out for Vista was something like "monitor for genuine microsoft software" (can't remember exactly what it was called), hwoever one had the option of installing this or not, but what it does is exactly that, it checks that computers connected to the net are using genuine (licensed) microsoft software and of not - well you get exactly what you are getting. There is no way of turning it off, and I think (but don't know) that after so long it could be that one day you get a blank screen and nothing else, i.e. they will stop the software being used. Or you may get a knock on your door (and yes they can trace computers, never believe that they can't).
Seriously, I would back your data up to an separate (remote) hard drive "just in case" that happens. You will need to anyway to get your PC rebuilt, as th eonly way out of this is to buy a new (licensed) operating system from Microsoft and get it installed.
Personally, if you can afford it, I'd cut your losses and get a new PC. A copy of Vista Home Premium from someone like eBuyer is £122. You would then also have to download or locate the various drivers for each peripheral device (although Windows should pull those down from the net as it installs in theory). A new PC these days is not that much more expensive and let's be honest Vista is not the best of operating systems, Windows 7 is so much more superior and is what Vista should have been in the first place. Me, I'd put the money I would have used to buy a genuine Vista licence towards a new machine.
If you do go that route, then you could continue to use your existing machine purely as a backup device for things like piccies etc - just don't connect it to the net. I still have my old XP laptop which I use for exactly that reason, loads of disc space on it, but operating system not maintained, not connected to net, so no updates - as there is no need, it is purely a machine for holding backups of my data. Also handy for holding soft copies of workshop manuals and microfiche when working on my bikes
Sorry rambled on there, but think you get the drift. Essentially you need to purchase a genuine copy of Vista and get it installed OR cust your losses, buy a new PC and transfer your data to it. You will not get rid of the message you are getting now and Microsoft are aware you are using a pirate copy of their operating system.
Howe