In March, researchers at the company identified 4,294 web sites and 89,860 associated web pages containing some form of spyware, up from 3,775 sites and 37,295 pages in February.
"The growth is faster than anything we've seen. These are all individual URLs that contain malware, basically," said Richard Steinnon, who heads threat research at Webroot, a supplier of anti-spyware products.
However, the number of corporate systems with spyware dropped slightly in the first quarter of this year, according to Webroot's report. The company found at least one some type of unwanted program - Trojans, system monitors, and cookies - in 87 percent of the PCs it scanned in the first quarter, down from 88 percent at the end of last year.
Instances of adware and system monitors dropped significantly but Trojan infections held steady, according to Webroot.
