The service pack for Vista is out since this morning. If you have Vista, you should upgrade to this instead of downgrading to XP. You may have to install updates that are listed as optional first before SP1 appears in the update list.
Vista has been out for more than a year and is more stable than when it first came out. In fact, unless it came with Linux or Mac OS X, PCs made from January 2007 should come with Vista pre-installed. Windows XP has been considered outdated since Oct 2004, but it’s predecessor has been delayed to end-2006 due to setbacks and the people behind it being busy with XP SP2 and Server 2003. I don’t see why people are not switching, or worse, downgrade.
The time difference between release of Windows 95 and XP is 6 years (98 and ME are somewhat like service packs of 95). But the time difference between release of Windows XP and Vista is 5 years originally as a minor upgrade of XP, which XP SP2 has taken over. Normally, minor OS updates are released every 3 years and major ones 6 years.
The next version is currently called Windows 7 (formerly known as Blackcomb and Vienna) due out in 2010. Do note that 32bit (x82) PCs are eventually going to be replaced with 64bit (x64) in the future. The former only supports up to 4GB of RAM while the latter can support more.