The setup file you downloaded does the installation on-line, personally I don't like this method (download link for the English version below). You are better to download the complete installation file so it can be installed off-line.
You should be able to install 5.0 over 4.8, that retains information about your existing registration and any period remaining in the registration period. It will then uninstall avast 4.8 and install avast 5.0.
However, if you wish you can do a clean reinstall (see ~~~~ below) but you would have to register avast 5.0 once installed - Registration avast5:
How to register avast 5 free on page 8
http://files.avast.com/files/documentation/quick-start-guide-free-en-ww.pdf also see
http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=459.
~~~~
- Download the latest version of avast, 5.0.677
http://files.avast.com/iavs5x/setup_av_free_eng.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear5.exe
find it here and save it to your HDD (it has uninstall tools for both 4.8 and 5.0).
- 1. Now uninstall avast! (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 4.8 if previously installed and then for 5.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.