What is istsvc.exe

istsvc.exe is part of ISTbar. This Internet Explorer add-in is spyware as well as a homepage and search page hijacker.

ISTbar may also install other parasites, including TinyBar, ToolbarCrash, SearchbarCrash, RapidBlaster and Download Plus.

If you don't recall downloading ISTbar, it may have found its way on your system by using an ActiveX control to silently install on your system by simply visiting a website. This is most commonly encountered on adult content websites.
You should remove it.