A fine recommendation from Elizabeth, no question about it! My personal recommendation would be to try Avast 5.x, which is the oldest version that still gets regular virus definition updates. Alternatively, you could also try Avast 4.8 if you want something even more lightweight in terms of RAM usage (and when you only have 512MB to work with you want to keep as much free as you can), but the last vpsupd.exe for that version was issued in March of 2017. Here are some links to help you get Avast 5.x set up/working:
http://sdfox7.com/avast/5.x/ (look for "setup_av_free.exe")
https://www.avast.com/en-us/download-update (look for "Avast VPS update for versions 5 to 8")
If you would rather give Avast 4.8 a try, then here are some other links:
http://sdfox7.com/avast/4.x/ (look for "setupeng.exe" and "vpsupd4.exe")
I am not sure if there are ways to get the newer 5.x-compatible updates working on Avast 4.8 or not. In any event, I hope this helps!