I don't think it's Avast asking you to pay. I think it's the program your mother downloaded and installed (by accident and unintended, no doubt). There are a lot of such programs, commonly called scareware or rogue; see http://en.wikipedia.org/wiki/Scareware for example.
How to get rid of it depends on what program it is. You didn't tell that. So please go to our malware forum http://forums.cnet.com/spyware-viruses-security-forum/ and make a new post with more details there.
My computer showed a virus, even though I have Avast Anti Virus installed. Avast then requested you buy the program in order to get rid of the virus. I thought this program was free, but it has essentially hijacked my computer. I cannot do anything on my computer - cannot delete the program, log onto the internet, or anything else. By the way, this happened on my mother's computer, not mine. I am thinking there is not really a virus, but they are wanting you to buy the program and they have hijacked her computer until payment is received. Could you tell me if this is normal, I have never run up against this before.