Windows XP. Microsoft have now stopped providing XP to retailers, so in a few months or sooner you will not be able to buy it unless from poeple who have stocks to sell.
Microsoft have set the EOS ( end of sales ) date to January 31st 2009, which has now passed. So what are the choices for new pc's? Linux in a variety of forms or Vista, not much choice really. Tell me what you think, I still sell it it on my ebay site because I still have stocks, but not for long.
Footnote to the above.
I am now testing a version of Linux as on the face of it looks and performs just like XP but without the hangups that soemtimes occur.All the desktop icons look the same, as do other areas which have a very similar appearance. Watch this space.