Oh hello Pam J. I misread your 1st post. Since you're running Windows XP, you install Windows Defender, and install Avast Free Edition. Avast Free Edition won't automatically disable Windows Defender. It won't cause conflicts between WD and Avast Free Edition, but from my past experience when I had Windows XP and I had AVG with Windows Defender enabled. WD is totally useless and caused many problems with it i.e. not detecting the malware that AVG detected, slow downs, and etc.
Nevertheless, it's all about the user and what they wish they want to do with having Windows Defender enabled and using it. It's all about the user and his/her discretion.
From personal and previous experience. It's best to uninstall Windows Defender, because it has poor detection rating, not enough protection tools and methods.