For me the OPs biggest problem is being on dial-up, not only would he have a hard time downloading any version of avast (or any other AV) on dial-up - he would have to get a friend on broadband do it or use a local government office/library to download.

But he is also on XP SP2 and SP3 is over 300MBs full download or an eternity on-line at windows update. Even then the monthly windows update for XP (until April 2014, clock ticking) is likely to be too much for dial-up. That the system is unlikely to ever be fully up to date and vulnerabilities may possibly be exploited, so you have to have your security up to the task.

So at the very minimum I would be looking at avast 5 or 6 downloaded by a friend/library along with the full virus definitions as if getting avast 5/6/7 from filehippo or similar site. The database is going to be way out of date and the first thing avast would do is try to download the complete database and why I suggest pre-empting the inevitably and download it on a fast connection first.

This system in itself with only 512MB needs I would say a minimum of 1GB for XP, the the AV isn’t the OPs only issue, just one of many that need to be addressed.