DavidR
8
The main issue is incremental updates and that would include virus definitions and program updates - you need to get to a reasonable start point (as up to date as possible) and then both incremental virus definitions and program updates would be so much hassle on dial-up.
As to if you should get avast 8 is a bit hard to say, ordinarily I would say get the latest version, but avast 8 introduces some other tools and your system with only 512MB of RAM (if that is anything to go by) is under resourced, the CPU is possibly the same one it came with Intel Pentium 4 at 400-533 MHz.
So avast 7 or 6 may be a better bet to start off with, check this site http://www.filehippo.com/download_avast_antivirus/13608/ this link is for avast 7.0.1474, when downloading use the FileHippo Mirror link (see image, click to expand), if you use the Download This Version button/link at the top of the page it will automatically download the latest version of avast (8.0.1489).