Do you get popus of software/websites for which you never subscribed or installed any application? That must be a spyware.
Windows anti-spyware is a security technology that helps protect Windows users from spyware and other potentially unwanted software.
This helps you to reduce negative effects caused by spyware including slow PC performance, annoying pop-up ads, unwanted changes to Internet settings, and unauthorized use of your private information. Continuous protection improves Internet browsing safety by guarding over fifty (50) ways spyware can enter your PC.
Supported by xp and windows 2000 series of operating systems
Download from http://www.microsoft.com/downloads/details.aspx?FamilyID=321cd7a2-6a57-4c57-a8bd-dbf62eda9671&displaylang=en#requirements
Good thing about this software, detects and lets you removes existing spywares and It will ask you before it lets any application installs itself permanently on your pc (like a shortcut on your desktop, a startup item etc).
Such complementary applications will make your operating system more reliable.