Personally I am sticking with Avast 4.8 a little longer, as the protection is more than adequate, for now. It's still an excellent AV; Avast 5 has more features and a new scanning engine.
The current non-beta version of Avast 5 can be found
Here.I've included some sensible steps and links below.
If you want to remove Avast 4.8, first get the full installer file for whatever replacement you decide on. Save that to your desktop.
Get the uninstaller for 4.8
here. Save it to your desktop.
Go offline. (You won't have a functioning AV for the next steps.)
Right-click the blue ball in the system tray, select "program settings>troubleshooting, and disable the self protection.
Uninstall Avast via the control panel.
Reboot.
Run the Avast uninstall utility by right-clicking and selecting "run as>admin". (A Vista peculiarity. Sometimes appears not necessary.)
If there were any problems uninstalling Avast the utility may need to be run in safe mode.
Reboot.
Install your replacement AV.
When it is successfully installed, go online again, get updates.
(The same general principles apply for removing/installing firewalls, regarding going offline for it. Make sense?)
Note, as DavidR mentioned above, Avast 5 free does not have a firewall. Keep Vistas firewall on, or use a replacement.