Strange behaviour - some text and e-mails disappeared

Two days ago a friend running Windows 2000 on a laptop with a wired
broadband modem connection was writing a Googlemail in Mozilla Firefox and
found text disappearing before his eyes. He also had a couple of e-mails
disappear from Outlook Express in front of him (and a later check showed
they weren't in the Deleted Items folder either).

Anyway, he disconnected from the Internet and ran a scan with his Norton
antivirus (well, if it didn't stop anything bad in real-time monitoring, I
don't suppose it was going to find anything on a scan). I got him to
run an online Kaspersky scan, which did find things that Norton had
quarantined and a dodgy e-mail he knew about and hadn't opened, but it
didn't appear to throw anything up that could be blamed for the odd
behaviour. (He's since removed - or is still trying to - Norton and
has installed Kaspersky.)

He runs free Zonealarm as the firewall. A few months ago he cocked up
and ran for a day or so with Zonealarm off and got a message from a hacker
to say he'd been hacked (nice to know). But since then had no obvious
problems. I got him to install AVG Antispyware and run a scan and keep
it running in real time. Again it found crap that shouldn't have been
there but nothing to account for the odd behaviour - or so he believes.

By the way, his Windows Critical Updates have always automatically been kept
up to date as has Mozilla and Internet Explorer.

Other than installing a new hard drive with a newer operating system, what
else should he do?

If we assume it is - and was - virus and spyware free, what assurance is
there that a hacker hasn't created/installed a backdoor method of entry?

Many thanks.

Martin


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