Having two anti virus is a very bad idea, you should always remove one before installing a replacement. Try deactivating both antivirus before running uninstall. If that doesnt work click here or click here may help.
you want to dispense with Avast in the first place. I've been running it on one of my own computers for some time, and as far as I'm concerned it's very good - it justs sits there and gets on with the job.
I'll be interested to hear about any problems you've had with it.