Well a 404 error is when whatever you are looking for is not found. So it could be many things.
Firstly it could be that your scraping proxies that no longer exist. Either from dead or bad sources.
Its also possible that your ISPs DNS is messed up, you could try changing to google dns.
http://code.google.com/speed/public-dns/
Your security programs such as anti-virus and malware checkers could be hijacking all the requests, or messing with scrapebox.
You could try different sources, try different DNS, try it in safe mode with networking support (if thats possible for you, cause offending programs won't load by default, then if that solves it you need to find the offending program and add allow rules or remove it). It could also be a virus.
ok, so what to do?
my antivirus catches no viruses.
I harvest and test proxies at the same time, so how they can not exist? Scrabebox harvests non existing proxies?
"safe mode with networking support" - not sure how - I am non-techie.
how to find the offending program?