I am using Mailwasher and today received lots (>20) messages with the subject line Undelivered Mail Returned to Sender.
My Outlook Express Sent folder doesn't show that I have been sending any emails.
Is this just normai junk email or should I worry that some virus is using my pc to send out emails?

Gary
You should look at the returned messages to be sure, but most likely a spammer forged your email address into the FROM: or the Reply-To: line of their spam email.

You are of course using good, current Anti-virus software to check your computer/software? If you have some malicious software on your computer, the outgoing spam, most likely, will NOT show up in OE. That would give you a clue something was wrong.

I'm endorsing Lou's thought that it is probably just your turn to have your address spoofed in a spam run. Depending on your ISP (and his inwards server filtering), there could be thousands more NDRs that you're not even seeing. Of the ones that are returned, they should all be showing the messages you supposedly sent as actually originating from different IP addresses if it's just email address spoofing. If, instead, it actually was a trojan on your computer sending direct to the internet, they would all be from the one IP address - yours. That would probably be the one you see when you visit show my IP. So, it's fairly easy to reassure yourself you're not 'leaking' spam. One hopes.

You can report misdirected bounces, just like you would spam. A slightly different report goes to the admins responsible, inviting them to mend their ways. Messages should never be returned to the email address shown in spam because it is almost always spoofed. They are incompetent if they don't know that and are in urgent need of education. They will get themselves onto blacklists if any of the addresses they bounce to are actually spamtraps.

SpamCop FAQ entry 'here' --> Why am I getting all these bounces? Updated!

SpamCop Wiki entries include;
spam Cop Top Ten Questions
The Quick Way to find Answers to your Questions
Forged From Address in e-mail headers
Misdirected Bounce
on and on ....

As noted in the previous replies and the FAQ/Wiki entries, this should not be considered "normal" e-mail traffic these days. Infection of your PC is another question entirely.