Microsoft has stopped providing support (updates and patches) for XP but they will provide anti-virus updates for Security Essentials until July 2015. Unfortunately, they have provided a nag screen for XP that shows at every log in.
To remove the notice the trick is to downgrade Security Essentials (it will still give virus updates) and then refuse to upgrade. Once refused, the security notices stop.
Superusers.com has a good tutorial with links. http://superuser.com/questions/738131/disable-the-security-essentials-end-of-life-warning-on-windows-xp
The following are five of the most reviewed free anti-virus programs available. One of them has been declared by at least one commenter as ransomeware and that’s something you don’t want to mess with, but I’ve included it anyway.
These are not in order of preference.
Avast! Essential (free) — at MajorGeeks.com (I trust them more that other download sites)
Bitdefender Free This the one where the commenter complained about Ransome Ware
AVG was in the list but it only has a free trial now, not free software