reinstall internet explorer because of weird behavior?

Question

I'm having a weird behavior with IE:

After some modifications on a Javascript enabled web page of mine, IE9 is not testing a string value correctly. I tried the same page on Safari and Firefox and it works (Who said no surprise?)...

Besides, when running locally with the Visual Studio integrated localHost engine, the modified page runs fine! Once the page is uploaded and run from the host website (ASP.NET 4.0) It doesn't "work" properly...

Now I tried on another PC with IE8 and... it works fine!

I suspect I have to reinstall IE9 on my PC?

Q1 - What do you web Gurus say? Q2 - How to reinstall IE on Windows Vista?

Update: the bad behavior happened after I added a string variable of 4 chars length and testing like this:

var a = "abcd"; // Actually a value taken from a hidden field

if (a[2] == somevalue) { Do this } else { Do that }

There is NO SCRIPT ERROR, but the result of the branching is wrong!