I don't think any PC manufacturers/retailers are now willing to sell PCs housing an operating system that is in it's 7th year these days. Only those small lightwieght "eee" notebooks may come with XP preinstalled.
Okay, Vista had got off on a rotten start at it's launch two years ago but it's grown up a lot since then and now that it's fully matured, it really isn't the "awful" operating system people have made it out to be. Let's face it, just about every new OS that got launched got slated (remember how Windows 98 crashed on Bill Gates on a demo machine just weeks before it's launch?! . . . *shudders*). Heck, I didn't hang about to install XP on my previous machine when that got launched, but didn't hang around for just as long to get rid of it as half of the hardware I had on me refused to work (graphics card, modem, printer and scanner went all dead under XP).
I've had Vista in my PC for a year and a half and once the updates and the SP1 is installed, it's actually a pretty decent thing to work with. Granted, it needed 4 GB of ram and a 1 GB graphics card to really get the best out of it, but I was going to carry out them upgrades regardless I've got Vista or XP on my PC anyway.
Most new PCs will invariably have the "matured" version of Vista pre-installed (including SP1 and all fixes and updates).
OR, you could wait until the end of this year, that's when Windows 7 may come preinstalled in PCs as standrad. But that won't be happening until December 2009 at the earliest.
Whoops, sorry if I went on a bit there but that's my take on things. And besides today's being one of those really quiet-I-haven't-got-much-to-do type of days.
PS: My post is based under the assumption on you planning on buying a new PC. I think trying to install Vista on a PC that's "on it's last legs" or threes years and older might cause more problems than good. For a start, you will need at LEAST 1 gb ram to even get Vista to run and that's with the aero interface turned off. Vista is really suitable when shipped with brand new PCs as it'll all be "ready made".