Currently most malicious software is designed to hide silently on infected PCs.  This study discusses findings from a recent study by Microsoft.

Microsoft MSRT Study on Malicious Software hiding in PCs
http://articles.techrepublic.com.com/2100-1009_11-6129235.html

QUOTE:  More than 43,000 new variants of such insidious software were found in the first half of 2006, making them the most active category of malicious software, Microsoft said in a Security Intelligence Report published Monday. In June Microsoft also flagged zombies as the most prevalent threat to Windows PCs.

"Attackers, with financial gain in mind, are clearly concentrating a significant amount of development focus on this category of malware," Microsoft said in the report.

Of 4 million Windows PCs found to be infected with some kind of malicious software in the first half of this year, about 2 million were running malicious remote control software, Microsoft said. The data is collected by Microsoft's free Windows Malicious Software Removal Tool, which runs when security updates are installed on Windows PCs.

While the number is high, it is actually a decrease from the second half of 2005, when Microsoft found that 68 percent of infected PCs contained a backdoor Trojan. Meanwhile, hackers are trying harder to make their networks of hijacked computers go unnoticed by moving to new Web-based techniques.