SiteHistory is an adware browser-hijacker that will give you all kinds of headaches.  SiteHistory was probably manually installed onto your computer as part of a bundled free software package.  It can also install itself by browsing through unsafe sites.  SiteHistory will change your settings and divert your searches to lead you to sites that the developers want you to visit.  It will also change your home page to a page connected to SiteHistory.
SiteHistory is malware and should be removed from your computer as soon as possible.  It will methodically slow down your computer, as it needs a lot of space in order to function.  In order to restore the health of your computer, you should find a trusted anti-spyware program to help get rid of this junk software problem.  SpyZooka is an excellent choice.