What are you trying to download?
Take my word for it you do not want Istbar on your system. It spys on you and would cause a ton of popups. It is a registered security risk
. See Symantec
for more information. Even if you can use that information to disable Istbar you shouldn't use software bundled with garbage like that. And if you do disable it you most likely have violated the license of the software you're trying to use. You might even kill it meaning it will cease to work.
Whatever application you want there's bound to be a better program for it. Tell us what you want and maybe someone knows a cool free or otherwise replacement.
You better hope you haven't already infected your computer,
read Read This
and post a log here. We'll look over it and if you still have some kind of malware, we'll have you repost it in the malware forum and they'll help you clean up your system.