What is the best free anti-virus/spyware protection available for Windows?

My anti-virus/spyware protection (which I stupidly paid far too much for) is about to expire. I know I’ve seen references to free downloads here, but am unsure which is the best for me.

Also, I understand that I need to uninstall the existing stuff before I download the new, is that right? How does one go about such a thing? If it matters, it’s CA Security Suite and I have Windows XP. Thanks!