Elietecandido welcome to Avast! Forums.
A little more info would be nice. Like:
¿ what does it say when the download/install can not be done ?
¿ What Windows are you running; XP, Vista, Win7, 8 ?
¿ Another security program actively running; AV, AS, FW ?
How to install Avast! cleanly:
1.) Very important
to remove all remains of other AVs installed before Avast. Use this site to execute uninstall utilities for other AVs.http://singularlabs.com/uninstallers/security-software/
2.) Download and save to your desktop aswclear.exe
3.) Download and save a fresh copy of Avast!. Be sure to have your lincense handy if running a paid version.http://files.avast.com/iavs5x/avast_premier_antivirus_setup.exe
4.) Open Avast! > Settings > Troubleshooting > Uncheck " Enable Avast! self-defense module ".
5.) Uninstall Avast! normally through Control Panel > Programs & feature ( Vista, W7 ) or Add and remove programs ( XP ). Reboot. If you can not complete this step, continue with step # 6
6.) Even if you were able to uninstall Avast! in step # 5, it is important to run aswclear.exe in safe mode for any Avast! you may have had before,e.g. v.6 o v.7, and then for v. 8. Reboot into normal mode after uninstall.
NOTE: aswclear has the ability to make your system go into Safe Mode. Just run it in normal mode and follow instructions.
7.) Reinstall fresh copy of Avast! and reboot.Be aware. Avast! will offer Chrome during installment. Remember to opt-out if you do not want it.