deleted DUN password

I have an intermittent problem on one of my PCs since loading Win XP.
Every now and again when I dial my ISP i get the message 'wrong password'
.
When I go to my connection setup, I find that both username and password no longer appear. After recompleting the setup config it's fine for some time, could be for a week or two then it happens again. Does anyone have any idea why? Do I have a virus that AVG doesn't recognise or is it an XP problem.
Thanks!

 

Have you deleted the connection, and recreated a new connection for your ISP?

 

dun password

Thanks for you message Paul. Every time the problem ocurred I re established a connection and, yes, it does stay stable for some time and then, it will lose its password and username. Therefore it is not a critical problem just a pain in the rear end.
Running spybot, adaware and AVG hasn't solved it but I will try your other suggestions. It could be a couple of weeks before I know that it has been fixed. Thanks again.