The updates for Avast 4.8 are still being released. However, I would note that 1) the 100MB+ update files are probably not dial-up friendly (all my machines are on TCP/IP), and 2) installing the latest manual updates can use up to 300MB of RAM, so on older machines, the update will likely take several minutes as the system will be paging to disk or using virtual RAM. Once the update is installed, you'll receive a confirmation (see Picture 3 below) and RAM usage will go back to normal.
I like the low overhead of Avast 4.8, on older machines it only requires a 486 with 32MB RAM. It's the last version for Windows 95 and NT 4.0. It also has none of the dependencies of the later versions, such as service pack prerequisites, which on later machines, SP's simply slow the machine down.
You can review the system requirements and installation visuals here: Picture 1 Picture 2 Picture 3 Picture 4