I disagree on having to get rid of old or small XP because Avast is "bloated".
Avast runs perfectly here, XP-Pro-Sp3 Pentium4, 1g RAM. Runs ok on Pentium3 with 500meg Ram, but that is slow by definition and xp-pro not even supported on it, though runs just fine.
Just defragment once a year or so, don't pile up zillions of security products, disable unnecessary windows services, don't let every installed program to run at startup, use CCleaner to get rid of accumulated trash and I hardly ever see more than 60-65% RAM use, 37% at most if not surfing the wild web.