Large fonts in e-mails received on Outlook Express

Due to ancient proprietary software that we can’t simply upgrade and that won’t run on newer PCs, my office is limping along on a lot of older equipment and software. Just recently, I’ve started encountering a problem with Outlook Express 6, which our server uses for our office e-mail accounts. Some incoming e-mail text displays in very large fonts. I’ve talked with the senders and they aren’t seeing this on their end. But if I reply to them, both my new message (which looks fine when I type it) and their original message come back to them in this very large font.

It might not be related, but I first noticed the problem shortly after running PC Tuneup 2011.

Guidance welcomed.