Not able to download-run WINXP & FIREFOX

Firefox crashed when I was doing a search. Then my computer WINXP stopped me from opening and EXE files. I cannot download any of the “workarounds” from the internet. Nor can I access the helpctr.exe file to go to a previous restore point. I can open Firefox, Thunderbird and my Avast Firewall, by doing a right click. Please advise soonest. Thank you, Bairnsmom