is my PC sending emails undetected

Just after paying a ~$300 bill online last week by debit card I got
about 100 email responses, relating to my address which had bounced
from various email boxes in Russia and Eastern Europe. They bounced
mostly due to full inboxes.
There is no record of these emails in my sent folder in Outlook
Express and since that event there hasn't been any abnormal activity.
Could these be reports for keyboard loggers ? What is the best way to
find out. I have since changed my email password. Should I change
my debit card too ?