The version of Avast you are using, although still supported for now, has been superseded by version 5; currently at 5.0.545.
Do you wish to upgrade to this version (recommended, by me and probably most others) or stay with the one you have?
What version of XP do you use (SP2, SP3?)
In general terms, you would download an install and save it (and download and save any other files needed, too) prior to uninstalling.
Whenever you are uninstalling/installing an AV (or firewall) and your computer is unprotected, it is best to be disconnected from the web during that time.
Security software installs and uninstalls should be run from an account with admin privileges.
The current version of Avast 5 can be found
here for downloading. (The Avast 4.8 versions can also be found lower down on that page.)
The Avast uninstall utility can be found
here.The procedure would be to:
- right click the tray icon, select "program settings" then "troubleshooting" and disable the Avast self defense module. (Opinion varies as to whether that is necessary or not. It can do no harm, and sometimes prevents the uninstaller from not working correctly from what I've seen.)
- Uninstall via control panel>add/remove programs. Reboot. (If there were problems with uninstalling, reboot into safe mode, see next.)
- Run the Uninstall Utility you downloaded. If there were problems with the uninstall, this needs to be run in safe mode. Reboot (into normal mode)
- Run the Setup file you dowloaded earlier, to install Avast. Reboot as prompted.
- Reconnect to the internet, get updates (will probably happen by itself.)
You should be good to go.