Tech help

Would one of you tech-savvy guys tell me how someone could send e-mail that appears to be coming from my user ID? Someone has been sending e-mail for a porn site under my ID, some of which bounced back as undeliverable (which is how I know about it). At first, I was thinking that my machine has a Trojan Horse, but I think my machine was shut down when the e-mail would have been sent. (How would I know a Trojan Horse is there?) It also has recently been swept by McAfee. The other day, someone also sent the same spam message over and over enough times to temporarily shut down my Hotmail account. I thought it was coincidence, but now I think that it's the caller. I had just written about the calls starting up again.

I should also contact my ISP and let them know what's going on (in case someone complains). Maybe they can change my ID.

I'll also definitely ask Erik to be involved. That is, my tech-savvy ex-husband who is a hardware engineer (hardware architect) with around 17 years of experience. He's known me since we were both 15, and isn't likely to be thrilled with someone messing with me, even though we've split.