In one of the first known cases, a scumware program has been classified as a true trojan. Symantec's Virus Center defines trojans and virii as:

Trojan Horse
A program that neither replicates nor copies itself, but causes damage or compromises the security of the computer. Typically, an individual emails a Trojan Horse to you - it does not email itself - and it may arrive in the form of a joke program or software of some sort.

Virus
A program or code that replicates; that is, infects another program, boot sector, partition sector, or document that supports macros, by inserting itself or attaching itself to that medium. Most viruses only replicate, though, many do a large amount of damage as well.

The CWS (Cool Websearch) trojan redirects random pages, many search requests, most mistyped URL's, and most antivirus site requests to one of several scurrilous and unethical search sites. The most common of these is the smartsearch.ws site.

Because this trojan spreads via a vulnerability in most Windows systems, keeping up with patches and security updates is very important. While there are no reports of this trojan causing true damage, it is annoying to have it hijack you to a site you didn't mean to go to. It's also just a clever code snippet away from having a potentially damaging payload added.