You can do a clean reinstall if you have saved your license.avastlic file that was emailed to you, as that will be required to register.
However before doing that - Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
If you did have another AV other than avast 4.8 for instance, then reinstalling may not resolve that problem.
- Download the latest version of Avast! IS 5.0.507
http://files.avast.com/iavs5x/setup_ais.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 (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.
- Avast! Internet Security Quick Start Guide
http://files.avast.com/files/documentation/quick-start-guide-is-en-ww.pdf