A google search for i-search turns up lots of sad stories. This is a nasty one.
But I'd still try downloading and running ad-aware and spybot. Avoid using other "spyware removal" software. These probably drop their own trojans, plus ad-aware and spybot are freeware.
ad-trojans in general and i-search in particular are programmed to "repair" themselves and prevent access to utilities like ad-aware and spybot. It may be necessary to download and install ad-aware and spybot while in "safe mode".
While booting, press F8 and a startup options menu will appear. Select startup with internet access (if running winXP). If you are running an earlier version you may have to download the software before restarting in safe mode.
Either way, once your in safe mode, only the minimum number of programs are loaded to run your computer; not the ad-trojan. You can then load/run ad-aware and spybot. These two programs may be successful in cleaning up your problem.
Otherwise you can resort to the heroics described on this thread.