I am also a WIN98 SE user (512 RAM and 55 G of free space) and installed Avast! Home Edition 4.8.1335 a couple days ago. Everything seemed to install just fine and I never ran into any problems except that I couldn't get it to scan my Outlook Express mail. The only two mail providers included were Outlook/Exchange and Internet Mail.
In the process of trying to resolve the issue, I uninstalled, downloaded again, and just finished reinstalling. NOW I have the same problem as Rogier. When I go to update, it jussssst about finishes before I get the evil red Avast alert that I don't have enough space to complete the task. I'm more than a little confused, as everything downloaded and has updated just fine until I uninstalled and reinstalled tonight.
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UPDATE!!
I uninstalled the download of the program I did from CNET and tried the one directly from the AVAST website. Still no luck. Then I remembered I had made my 'working' download a few days ago from filehippo. TADA! I don't know why, but the AVAST program (still most recent) from filehippo works with my WIN98 SE. Like I said, I don't know why... and don't question it... just know it works. Now I'm onto figuring out how to make it scan my Outlook Express mail. GOOD LUCK, Rogier!
I'm anxiously waiting for a suggestion to successfully resolve this problem as well as to get Avast to scan my Outlook Express mail.
Thanks in advance and keeping my fingers crossed!