I assume you are using avast 4 home and not the avast 4 server edition (the forum you have posted in) ?
Are you getting any error messages when you try to update ?
What is your firewall ?
You said you couldn't uninstall avast, what was the problem, errors, etc. ?
Download the avast! Uninstall Utility,
find it here and save it to your HDD so you can find it later.
If you can't use the add remove programs try running this.
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall (or run the uninstall utility), reboot, install, reboot.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.