It could be because you have way too many items in your startup, a highly fragmented hard drive, too little hard disk space, invalid registry entries, outdated drivers or software, unnessary services starting automatically, a security software taht uses too much CPU and memory.

Or you could have Malware.

Malware, short for malicious software, is software designed to secretly access a computer system without the owner’s informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code. The term “computer virus” is sometimes used as a catch-all phrase to include all types of malware, including true viruses.

One type of malware has emerged in spyware — programs designed to monitor users’ web browsing, display unsolicited advertisements, or redirect affiliate marketing revenues to the spyware creator. Spyware programs do not spread like viruses; they are, in general, installed by exploiting security holes or are packaged with user-installed software, such as peer-to-peer applications.

I use a free software called MalwareBytes with good sucess.

Malwarebytes Anti-Malware is a surprisingly effective freeware antimalware tool. It’s a relatively speedy malware remover, with the quick scan taking about 8 minutes even with other high-resource programs running. The heuristics engine proved on multiple computers during empirical testing that it was capable of determining the difference between false positives and dangerous apps.

The app has some nice features rolled in, too. It supports multiple drive scanning including networked drives, context menu options including a scan-on-demand for individual files, and the FileAssassin option under the More Tools section for removing locked files. The interface is simple, but pleasant-looking and well-organized. Tabs live just below the oversized logo, with few options per tab to keep down the clutter. The installation process was fast enough, but interestingly offered up the well-kept changelog and an instant definition file update.

Do note that the real-time protection is restricted to the paid version, as is the scheduler for updates and scans. Overall, though, Malwarebytes Anti-Malware is a responsive malware remover that does what it should with a minimum of fuss.

For more information go to MalwareBytes.org