This is almost certainly not a problem and not connected with the email problem either.
The bounced unsent emails are usually the result of the computer of someone else who has your email address in their address book being infected with a virus. Emails are sent out in your name, so you get the bounces. These usually die out after a week or two, and meanwhile there is nothing you can do about it.
The changed MBR is more likely to be caused by your computer having crashed and the MBR getting out of sync with the files on your computer. This is common with Windows 98 computers when they have bad shutdowns, and the user cancels ScanDisk which is trying to correct the error. Restoring the MBR SHOULD put things right.