From January 30th, all new PCs will come pre-installed with Windows Vista. There are several good reasons not to use it.
If that article's a bit long and technical for you, read on...
Vista is all about 'premium content'. This is music or video that's copyrighted by one of the major music or movie companies. Basically, if any such content is being played, Vista goes into a sort of 'protected mode'. When this is happening, your computer becomes very careful not to 'leak' this protected content. That means that any devices on your machine that can play this content (eg. your video card, sound card etc) have to work in a special way. The idea is (for example) that your video card will send coded signals to your monitor, so that you couldn't plug in some sort of recorder and simply scoop up the signal and record it. The problem is, not all monitors/TVs can handle these coded signals. If yours doesn't then Vista will reduce the quality of the output so that it's not worth recording. Incidentally, you don't get a choice, it just happens.
Futher than that, if you're listening to a CD, then your monitor may go fuzzy at the same time! Just because one thing is handling protected content, the other may become crippled as well!
As if that's not enough, some devices can simply become disabled, either during premium content playback, or at some arbitrary time in the future as decided by Microsoft. Basically, if a device (or it's associated driver software) are found to be leaking content sometime in the future, then Microsoft can stop it working when you next update your computer. Given Microsoft's history in this area, it's pretty much required to update your computer say, once a month at the moment, so you're almost certainly going to *have* to get your leaky device disabled reasonably soon after MS decide to disable it. They'll disable things until some fix can be found, provided one can be found. Maybe the hardware/driver in question is so old the vendor doesn't want to fix the problem, or maybe it just can't be fixed.
This is horrible: You update your computer to protect yourself from viruses and such like, and instead your graphics card stops working! If it's one built onto the motherboard, perhaps on a laptop, then well, you're screwed - go and buy a whole new machine, as it's almost certainly not worth trying to fix it.
So to summarise these few points (yes, there are more I haven't even mentioned!):
1) All Vista compatible hardware will be more expensive than it otherwise would be. This is because hardware vendors have to comply with very strict rules, and have to add extra hardware features to their products to make them work with Vista. This all requires extra testing, which aside from costing more takes more time.
2) When playing premium content, your computer's performance may degrade. That is, the video or sound quality in particular may degrade, but also raw system performance may also degrade as your computer works to protect that premium content as it moves around inside your computer.
3) Whatever your computer is doing today, it may not do it tomorrow. If you're unlucky enough to have some hardware which later is found to be unfriendly to the content industry, it may just aribtrarily stop working. It's not because of a hardware failure per-se, and you may not even know how, or want to copy premium content, but Windows will refuse to use your device. If you're really lucky, an update may come along in the future, if not, you may well end up buying a new computer (oh, and your old one probably won't fetch much on Ebay either!).
I don't think Vista sounds like a good deal for end-users. Microsoft look set to do quite well out of it, provided we all lap it up like we're supposed to.
Update 14th Feb, 2007: A little looking around brings more to light about Vista's hardware requirements. Reghardware started me on this trail, but arstechnica have a slightly more inclusive article along the same lines.
To me, these articles say very loudly: Don't buy a Vista PC until AT LEAST June 1st, 2007, and preferably June 1st, 2008. Only then does the hardware you buy fully reach maturity, and so will be optimal for running Vista. Before that, you'll always have hardware that doesn't do everything Vista wants. Given what we know about Vista and it's use of hardware, I'd say anything less than optimal hardware is an incredible risk to take. Annecdotally, by June 2008 it's also likely that there'll be a Service Pack or two out, so the major problems with Vista will have been fixed.
So by Vista if you must, but wait until June 1st, 2008 to do it.