I am using Windows XP from long and never thought of upgrading to Vista for a simple reason, for me XP works well and there is no point in upgrading to Vista after reading so many articles on how Vista sucks. So what is next as XP retires, few days back i asked the same question myself and immediately got an idea, why not try Ubuntu.
My laptop is 3 years old Sony VAIO with 80GB Hard disk and i used most of the space, so i cleaned the system by deleting some files and transferring some to external hard disk and made some space for Ubuntu. This is the first time i am using Ubuntu Linux (i used RedHat enterprise linux few years back).
Ubuntu installation is smooth, get a live CD and boot it. If you want to dual boot windows xp and Ubuntu follow this tutorial.
Ubuntu is really good OS and it comes with some great stuff like OpenOffice, GIMP among others. Agreed OpenOffice and GIMP may not be as perfect as MS Office and Photoshop but they are not bloated like their counterparts and the features they have are enough for most of the users. Best of all they are open source software and comes for free.
Now i am using both Windows and Ubuntu. I may shift completely to Ubuntu Linux after some time. If you have never tried Ubuntu try it, it is really a good OS and best of all it is free.