Gmail sending virus?

Hi , I got calls from a few friends recently asking me why i was sending virus from my gmail account. i was shocked so i scanned my pc and it did detect a virus . by the time i noticed the virus name the dialog was closed. after that i formated my hdd and changed my gmail password. since then there was no complaints. Today i got a call from my cousin telling me about the same thing . now i am confused as my virus defs are up to date and spybot SD detected only a single tracking cookie( i dont remember the name it was some thing like "A... A , Inc" ). I updated my defs and did a full system scan it did detect One Trojan (dont remember the name). . recently i recives some mail from my friend and my AV detected it as blackmal immediately and deleted it. I have configured opera as my email client so i seldom use the web interface.Im now wondering if my pc is sending a virus or any one doing it. Is this a known problem ??

I use WinXP Pro with ZA security suit as my AV and firewall.

thanks
Charan

EDIT:oops It did detect a single trojan