DavidR, Ardvark
Thanks for your suggestions. Here is an update (still not fixed):
1. I have run hijackthis and saved the log file.
2. I looked at the instructions that they want me to go through before logging the file and it looks very involved. I will do it, but I need to clear some time to do it. It looks to me that it will take some hours to do.
3. In the meantime, I copied the file ashavast.exe (from another PC I have that is running avast fine) to the avast4 directory, and this time I saw it magically disappear before my eyes. There is definitely something that is seeing that file and choosing to delete it.
4. I have looked for the ashdisp.exe in the avast4 directory and it is not there either. I think that it is being deleted by whatever it is as well.
5. I did have other antivirus software on this PC before and I think that I got rid of it, but maybe I didn't get rid of all of it. However, I can't see anything in the running processes that is antivirus software. I assume that there would be a running process if there was some old antivirus software running?
6. I will need to reboot and re-install avast again to see what avast processes are running (there are none now). I think when I looked previously there were a couple running, but I will check and post again.
7. One process that is (was, as I stopped it) running is hidn1.exe. I have no idea what this is and my sense tells me that this is a virus/worm or whatever. I cannot find any reference to this file when I search the hard disk. Do you know what this might be? However, after stopping the process, the ashavast.exe file still disappears when I copy it into the avast4 diry, so I guess this isn't the problem.
Thanks for your help. I would love to find out what is causing this. I really believe in avast, and I want to use it!
Cheers
Alec