I hate change and therefore only upgraded from Windows 2000 to Windows XP a few years ago.
My old Viglen which ran XP had recently got to the point where it took at least 5 minutes for Mozilla Firefox to load after clicking on the desktop icon and even for a patient person like me that was too much. It couldn't be helped though as the hard drive was over 90% full.
I did actually buy a brand new laptop (Emachines E520) back in February 2009 which had Windows Vista Home installed. Despite the negative reviews of the operating system I never had any issues with it whatsoever and despite getting rid of it in 2012 due to the laptop itself failing, the OS worked fine right up to the end.
Therefore when I decided a few weeks ago to replace the desktop PC I opted for the most cost effective option - a Dell Optiplex 760 (Ultra Small Form Factor computer) with Windows Vista Business. The price was under £40 which sold it for me immediately.
All I have to say is the memories I have of Vista are still 100% true, the OS is very fast, smooth running and reliable.
At work we use full size Acers and HPs with Windows 7 installed and not one of my colleagues has not complained about the slowness of the operating system. Personally, my work PC is almost as slow as my Viglen I just got rid of and that was manufactured in 2004 with Windows XP!!
I'm not saying Windows 7 is bad, on a higher performance PC it may be brilliant - but if you are looking to buy a low spec PC then in my opinion Windows Vista is the best OS to install even in 2014.