I have seen this problem on another site, the only difference seems to be that in the other case, only Yahoo is affected, once the problem has been encountered it also affects Internet Explorer.
Since he also has AVG plus Zone Alarm on Vista, and his other PC's running on XP don't have any problems, I and suspect of the combination of AVG plus ZA on Vista.
He has decided to run with IE instead of FF for now. But it might be worth disabling both ZA and AVG (for a short period only) and see if things are OK. You could also try replacing AVG with Avast. It would be OK to run for a while without ZA, use the Vista firewall instead.
This is going to be trial and error .......
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I realised I was dyslexic when I went to a toga party dressed as a goat.
