According to research by security vendor PC Tools Software, Vista is 37 percent less susceptible to malware than Windows XP but is more vulnerable than Windows 2000.Based on statistics gathered from users of PC Tools' ThreatFire anti-malware service, Vista let through 639 threats per 1,000 computers, compared with 1,021 for Windows XP but only 586 for Windows 2000. The report didn't attempt to quantify what proportion of malware targets XP compared to Vista.

A Microsoft spokesperson questioned PC Tools' methodology: "We appreciate independent studies...however, this is a study by a vendor of anti-malware products. ThreatFire vulnerability comparison numbers certainly don't reflect our vulnerability findings from the malicious software removal tool, which ran on over 400 million machines in December 2007."InformationWeek