For the avg user that only needs internet, office, and music/movies Windows XP is good enough. What does Vista/7/8 offer a person using a Pentium 4 level computer? Nothing really worth paying for. I am surprised MS doesn't get more credit for XP which could be seen as a relativity dependable workhorse. Problem is for MS of course they ended the need for the users to upgrade.
Windows XP extended support ends in April (2014). Having your computer hacked due to unpatched OS and browser should be worth paying to avoid. If you don't feel like paying, there's always linux distros, *bsd, nexenta OS... http://support.microsoft.com/lifecycle/default.aspx?LN=en-us...
I've been running unsupported XP SP2 on one of my older machines for over a year now. It's connected, runs applications, email, and internet services. No hacks, no worms, no rootkits, no nothing.
Just use a good antivirus and you'll be fine. AFAIK there hasn't even BEEN a 0 day patch for XP in more than 3 years.