What is your operating system, the cpu, and the RAM of this computer?
What Avast version? When was this installed?
What AV was used before Avast was installed?
Sometimes a complete re-install can solve these sorts of issues. But do try delaying the loading of the Avast services, first, or also highly recommended is to trim your startup list (programs that auto-start with Windows, which in many cases don’t need to) as recommended by craigb.
The specs should enable a fast-running XP, so you probably do have a lot of programs starting with Windows. Worth checking.
For a full reinstall I suggest:
-Download the latest full install file from Avastdotcom and save it to your desktop, or a purpose-created folder.
-Download the Avast Uninstall Utility and save it.
-Go offline.
-Uninstall Avast using Add/remove programs. Reboot. If there were any problems with uninstalling, reboot into safe mode.
-Run the uninstall utility. Reboot.
-Install Avast from the file you just downloaded. Reboot.
-When the icon shows normal in the system tray, reconnect to the net, check for updates, re-register.
A quick reminder that if you decide to reinstall avast make sure that you run the removal tool that Tarq57 has refered for the 4.8 and 5.0 versions after you have deleted though add remove programs