hacked email?

A few months ago I got a virus - I believe it was a trojan horse program. I had been using my laptop at college and using the unsecured wireless network. I didn't have time, between work and school, to bring my laptop to get fixed. A friend told me to set it back to when it had worked okay, so I did that everytime it gave me problems until I could get it to the computer shop a couple weeks later. I was told that was a mistake because it allowed the virus to burrow deeper into the operating system. Anyhow, the computer was cleaned up and I bought Kaspersky Internet Security to protect it.
I still have had to use the laptop at school, on that same network. Last week I found out that someone used my email account to send spam emails - they were in the sent folder the next day. I emailed everyone to warn them not to click on any links, and then went to change my passwords on every thing I could remember that has a password. I used my desktop computer for that. That computer, although I bought Kaspersky for it, is still running Avast because I haven't taken the time to install the new software.
I talked to the computer guy today, and he told me it likely has nothing to do with the network at school, but I probably have a worm or virus that got my password. I use four different computers based on where I am at what time - I've checked out my laptop (run a scan on Kaspersky and also ran mbam) and have a scan running on one of the computers at work. I plan to scan the desktop and the second work computer ASAP, but am wanting to make sure that this is likely the problem, and not the network. I guess I'm wanting a second opinion, and some recommendations if I should be doing anything else.
Thanks,
cranbearie

 

Just to clarify - I am not running two antivirus programs. I still have Avast on it because I haven't had the time to take it off and put Kaspersky on it. I do not know that I changed the passwords from a clean computer - I do not know which one has a virus. I use four, and of course none of them has told me I have a problem. I am still wondering it is possible someone got my password through the unsecured network at college.
I am at work now, on a computer I cannot do anything about if it does have a virus but I rather doubt it is this one because the IT people are fairly vigilant about updating and whatnot on. When I get home, I will run through the malware removal guide on the two computers at home.
Thanks.