While XP was around for about 5 years, as rojash says, the next version of Windows will be out in October so Vista will have been around for about 2 and a half years.
Normally you would expect the next release of Windows to be an upgrade to Vista (but keep the name).
Changing the name of Windows is a very expensive process, so the fact that Microsoft are getting rid of the name Vista (so it seems) makes you realise they think it has been a disaster for them.
Microsoft rely on companies upgrading their version of Windows (that way they can charge for hundreds or thousands of licenses) but the uptake for Vista has been very poor inside large companies, hence the rename and relaunch.
I think Vista has been one of Microsoft's worst public relations disasters (if you include the huge delay in them shipping it).
They will be glad to se the back if it.