"Trojans" are just one type of virus. Any "file" that attempts to "replicate" itself and/or send copies of itself to other machines is a virus. A "Trojan" is just one that installs itself and waits for a while before doing something, instead of immediately trashing things.

The have been more than a dozen new versions of just ONE new virus form that first appeared less than a month ago. Some individual sites have reported that, sporadically, up to 15% of their email traffic was being generated just by variants of this one worm, attempting to send itself to infect more machines during the past couple of weeks. (SPAM mailers accounted for more than 60% of all email on all sites in February, so legitimate users only get 25% as a leftover?)

There are a few thousand identified viruses in circulation on the internet. Any slimeball with a 6 week course, or a few hours at the right sites, can "modify" any virus and restart it, although most of the "copy and paste" a-holes won't change the "signature" enough to cause a major problem.

They do evolve. If you use email, or browse the internet, you MUST have a decent AntiVirus installed, and you MUST update the "signature files" at least weekly if you expect to maintain reasonable protection from this crud.

John