Microsoft's latest security intelligence report shows a resurgence in worms, although rogue security software also remains a big issue.
Rogue security software was found and removed from 13.4m machines, compared to 16.8m last time. It is still an issue but numbers are falling.
Worm figures doubled in the first six months of 2009 - from fifth to second. The focus on worms is partly to do with attention given to Conficker which infected 5.2m machines.
Taterf doubled to 4.9m compared to the second half of 2008.
Taterf is a worm aimed at massive multi-player games. It spreads via USB drives and mapped drives. Surprisingly it appears in enterprise space rather than consumer space - presumably by people sticking USB sticks into work machines.
Taterf tries to steal the assets you have within the game - which has a value to cyber-crooks. This is less of a problem in the UK but China and especially Brazil have very high levels of infection.
In fact, the UK comes out looking fairly positive, according to Microsoft. We have our share of worms but far less than elsewhere. This is due to better protection and because consumers are more likely to use automatic update processes than those elsewhere.
Cliff Evans, head of security and privacy at Microsoft, advised consumers to keep automatic updates on, keep a firewall running and use one of the newest browsers and up to date anti-malware. He said it was important to check all your software, not just Microsoft's.
For Taterf - never log in to your account unless you trust the machine and be suspicious of downloading short cuts or cheats.
Businesses should follow the same advice. MS security update guide has more detailed advice for IT professionals. Systems Centre allows you to manage all security updates and not just Microsoft's.
Looking forward, it appears that worm infections will continue to grow. Evans said: "if criminals keep making money from it they'll stick with it, but there are tools to protect yourself against all these threats."
Microsoft works out the infection rate per thousand machines. The worldwide average is 8.7, Japan, Austria, Germany run at about 3 and the UK 4.9, down from 5.7. In the US the figure is 8.6.
The top worm in the UK is koobface which spreads via Facebook and MySpace. It has been around a while but infection is increasing.
Microsoft publishes this report every six months. It claims the data provides the most comprehensive snapshot of internet threats. Information is collected from monthly Microsoft updates, which look for any infections on 450m PCs every month. Data is also collected from OneCare and Windows Defender, which run on 100m machines.
Microsoft's Forefront Online Protection for Exchange - scans billions of emails.
Finally Bing scans billions of web pages every year.