The UK has one of the highest rates of computers infected with secret programs that can track what people do with their machines, research shows.
Nick Palmer: "Spyware is now the biggest danger"
Only Thailand and the US had more infected PCs, found a study by security firm Webroot.
It was released to coincide with a meeting in London of MPs and computer experts to discuss what can be done to combat so-called spyware programs.
These hide themselves on hard drives and can gather information about users.
According to anti-spyware firm, Webroot, almost 55% of consumer Windows PCs are infected with so-called adware.
This is a form of spyware that can subject people to pop-up adverts, hijack their homepage and install bookmarks.
STAYING SAFE ONLINE
Install anti-virus software
Keep your anti-virus software up to date
Install a personal firewall
Use Windows updates to patch security holes
Do not open e-mail messages that look suspicious
Do not click on e-mail attachments you were not expecting
The most malicious spyware programs lurk unseen on PCs and steal confidential information such as passwords or login details.
"People were worried about spam but spyware is now the bigger danger," said Dr Nick Palmer MP, who sponsored the meeting at Westminster.
"Previous ways of computer harassment, from viruses to spam, have been countered too slowly causing endless trouble for ordinary computer users.
"This time we need to stay ahead of the curve."
Spyware and adware often travel alongside file-sharing programs or media files that people download from the web and targets Microsoft's Windows operating system.
Some websites also take advantage of bugs in Microsoft's Internet Explorer browser to install spyware on the machines of people that visit.
Free anti-spyware programs such as AdAware and Spybot are used by many people to clean up their PCs.