Script running makes browser run slowly

I have started getting an error message during loading of my index page which says something like:
"a script on this page is causing Internet Explorer to run slowly. If it continues to run, your computer may become unresponsive.
Do you want to abort this script"

It makes no difference whether I click yes or no, it slows down to an unbearable slowness. I loaded Firefox to eliminate the problem. Same thing happened but to a lesser degree. I also wonder if anyone at your end can play around with users adding unsolicited scripts. If so, I really would like you to eliminate their leisure to do so and would like to know if I can do anything from this end. If not, I may have to evaluate the risk of building my sites at Synthasite. I'm not a computer geek so I have to trust you guys over there to take care of our security and ease of work.

Thank you in advance.
best regards
ayhan