AFTER BEING SURE to have *completely* removed all traces of previous
programs I no longer wanted, opt for Avira Personal A/V as main real time guard/AV. Light on resources and pretty effective.

With the free version each update brings a pop up to upgrade to paid but other than that pretty good. Officially works XP SP2 & above but works fine on my old XP SP1 as well. Updates daily Mon-Fri inclusive.
New program versions auto install over old version via updater.
http://www.free-av.com/en/index.html

MBAM & SAS make good free on demand scanners for malware/spyware.
I still have a soft spot for Spybot & it does sometimes get stuff others missed. Enjoy!! Happy