Before you run HijackThis,

Get updates for your Antivirus and AdAware/SpyBot etc.

Boot into safe mode and empty your browser cache and history. Go into Windows Explorer and find your Windows Temp file..delete everything it will let you (it only moves them to the recycle bin).

Now run your antivirus, antispyware, antitrojan etc software, also in safe mode.

Then reboot and run HijackThis

You might as well get rid of as much rubbish as you can before setting about the stubborn ones