A clean reinstall might be best ensuring all remnants of previous installs are removed:
- Download the latest version of avast, 5.0.507 http://www.avast.com/free-antivirus-download
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 (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.
Direct download link if needed - Free AV: http://files.avast.com/iavs5x/setup_av_free.exe
- Documentation, Avast! Free antivirus Quick Start Guide http://files.avast.com/files/documentation/quick-start-guide-free-en-ww.pdf
Given your though that there might be malware already on your system, it could possibly interfere with the install (but I don't believe that error message relates to that). So it may be helpful to also run these tools.
If you haven't already got this software (freeware), download, install, update and run it and report the findings (it should product a log file).
Don't worry about reported tracking cookies they are a minor issue and not one of security, allow SAS to deal with them though. - See http://en.wikipedia.org/wiki/HTTP_cookie
Also available a portable version of SAS, http://www.superantispyware.com/portablescanner.html
, no installation required.