This is most likely due to system instability created when a malware integrates tightly into the operating system, making it difficult to remove.
As this problem maybe specific to your system, you should follow the steps in "My Computer is Infected, what
should I do?"
Here is a list of malware known to cause this problem including a general description of its behaviour
- Virtumonde (a.k.a Vund Trojan)
- Annoying Winfixer pop-ups when browsing (Sysdefender the latest incarnation)
- Opens site to www.amaena.com
- Redirects searches from address bar
- Solution: Lavasoft is currently developing a generic Virtumonde removal tool, but for now, you should follow the comprehensive instructions from DSL Reports here