Windows XP / mail problem

Hello,

I hope nobody actually HAS this problem, but maybe someone HAD it..

On Windows XP installation, with updates, Adobe kit (photoshop, InDesign, Illustrator), OpenOffice, a problem developed: e-mails cannot be sent using this computer.
E mails are actually sent, but rejected by the pop3(!) server because of embedded NULL characters in the mail. Actually the SMTP server handles the e-mail (they are probably different software - pop3 and smtp..).
This information was received from internet provider after some discussion with them.
As a result, e-mail can be sent only to some addresses, but not others.

We have tried different e-mail clients: MS outlook express, MS outlook, Eudora, Netscape mailer.

ALL of them seem to have developed this embedded NULL characters problem.

Any hints (before we format&reinstall - with success not guaranteed)?

Thanks really,

Relative