Pardon me if I sound callous, but I'm assuming you don't go on vacation and leave all your doors and windows wide open. Why would you do this with your computers?
Since you are questioning the possibility of a virus, this tells me you probablyaren't running any anti-virus software. You don't mention any firewall, so I'll also assume you don't have one.
If I understand you correctly, you are getting undeliverable (bounce) messages which report that email purportedly initiated by you can reach its intended recipient. Is this the case?
It is more likely that a Spammer has hijacked your email address and is using it as the return address, than it is that you are actually sending out all these emails. You can probably verify through Hotmail whether or not the original messages were initiated by your computer.
Having said that, if you have have a 24/7 Internet connection, and you leave your computers on all the time, and you have neither a firewall nor anti-virus software, there is nothing protecting your computers from the undesirables that skul around the Internet looking for easy prey, and anything could be possible. A hacker may be using your computers without your knowledge to send these emails. Any personal information that exists on these computers is free for the taking by these same undesireables.
First, get anti-virus software for all your computers. Run an immediate scan. Keep the virus signature updated. The anti-virus software will prevent your computers from becoming infected.
Next, get a hardware firewall. These are available (in the form of a DSL/Cable Router/Firewall) from many vendors and can be found for less than $75. The firewall will put a lock back on the doors and prevent your systems from even being seen from the Internet.
Good luck, and please practice "safe Internet!"