It cut my system speed in half and decided that nearly every program I had was a virus, then it would hang the system. The uninstall did not work either because it thought it's own uninstall was a virus. uninstall in safemode did not work either. I had to use my tablet to download a cleaning tool, then transfer to my cell phone via bluetooth, then use cell phone to plug into computer and get the remover tool.
A lot of effort.
Nothing even remotely like that has ever happened to me, or to anyone I know who uses Avast!, and the fact that it thought a part of it's own program set (the uninstall) suggests it wasn't Avast! as such at fault, it's much more likely that either Avast! (or a system file(s) that Avast! calls) has become corrupted or an infection had existed on your machine prior to you installing Avast!, and the infection had infected Avast! before it was run for the first time (and so before it could detect and remove the virus).
You really should run scandisc, then (on a machine you know to be uninfected) download a bootable anti-virus disc (AVG or similar) and boot it on your PC either from DVD or USB stick, and scan every singlefile. Doing this from a bootable media will negate any virus that might already be in your system. --- I'm a PC and console gamer - I love good games, regardless of their host system. For all things N64: http://z9.invisionfree.com/Nintendo_64_Forever/
Currently using Kaspersky with Malwarebytes free on a similar SSD bootable and user files on HDD setup, Windows 7. Works beautiful, no problems. Once in a while, Kaspersky picks up a suspicious website. I'd recommend giving the trial of Kaspersky Internet Security a go.
Always hated Norton. Even when you uninstall, remnants of it linger and annoys the crap out of me.