I had recently opened a link i thought someone had posted me while chatting on msn and suddenly ended up with a trojan horse stuck on my computer. While it was on my computer and before i could entirely remove it I had sent the same link to all my online friends without knowing it. After that, every time i opened firefox i would get loads of advertisements in a new tab.

Has anyone else encountered this self-spreading virus?

yup but aint never clicked a link...i just see people sending them usally saying

is this picture of you?
http:// blah blah virus.com

or something similar
you click and BAM you fell for the same trick as them...anyway if you didnt downloading anything it might just be a cookie or temporary internet file

yeah

1) get better friends who dont go on msn when there infected
2) encourage such friends to get AV and keep patched and have the most recent version of live messenger
3) keep patched yourself

and most importanty,

4) dont click anything!

if a random guy came up to you in the street and asked if you wanted some chocolta you would say no, right? same thing on the net.