Although possible, there shouldn't be any difference with Opera Mail between WinXP and Win7.

Did you move your mail folder from WinXP to Win7? If so, there might have been some file corruption where it was exposed when you moved/copied the mail files.

Run a database check for starters.

Also, click the wrenchicon on the mail panel toolbar and reset the mail panel to defaults. Then, click the "settings for this viw" icon on the toolbar above the "All Messages/Unread" message list and make sure "show trash" isn't checked.

Is this an IMAP or POP account?