I’m not going to make up an answer for something I’m not familiar with, such as the nagscreens in avast! 4.x, so yes, I’m not answering that.

Regarding the installer… I admit the message is “slightly” confusing, but it should be understood as “The system doesn’t meet the necessary requirements. Besides, btw, this program cannot be installed on a server OS”. Since the penultimate line in the log checks the version of Internet Explorer, I’d say that’s what actually has failed (but I’m guessing according to the truncated screenshot and not knowing whether you might have uninstalled IE completely from the system, and also not knowing what exactly the installer is looking for, as it’s missing from the screenshot).

Btw, the layout of the virus database has been completely redesigned and optimized in version 5/6, so the new versions certainly use less resources than version 4.x - both in CPU and memory terms (i.e. more suitable for older systems especially). OK, the UI is probably more hungry than in v4, but the background operations (updating, scanning, …) are definitely lighter.