I would have you try something else, but don't have the authority to instruct on this, not being a member of the Malware Response Team. Nor do I want to assume the responsibility of possibly torching your XP install, though if I were doing the work, I would take the chance. However, it's not me & don't want to violate Forum rules by instructing you on advanced methods that *may* in unskilled hands do more harm than good.
Being that you can't update Windows Defender, execute any .exe files, something *may* be up here. From your posting, it looks as though you've yet to install security & still in the process of setting things up. Unfortunately, for XP, updates comes from IE6 at the beginning, the least secure version of IE of the modern era, has existed since 2001.
My suggestion to you is to post this in the "Am I infected" section of the Forum. The computer may not be infected, however I'm inclined to believe that there's the possibility, based on what you've stated. Here's the link to that section.
I would like to further assist with this, but again, my hands are tied. Unless the MRT states it's not an infection.
Best of Luck.