[Javascript] Debugger

[Dave Stoltz]

I have a stupid question:

Is there something I can use to debug javascript?

I'm running XP, and 1 of 2 things happen when I encounter a Javascript error
on my page:
1) I get a popup that tells you the line, and the error description - but
doesn't allow to debug
2) I get a popup that if you click YES, will open some kind of built in
script editor that allows me to step through the script and debug it to some
extent.

I can't figure out why sometimes I get #1, and why sometimes I get #2
popup...

Is there something off-the-shelf I should be using?

Thanks for any direction!