I downloaded Gutsy with IE7 & 6 and Firefox overnight on my laptop. No
problems. All three at about the 25% mark stopped for a while but then
started again. You are dealing with you own computer, your physical
connection, your isp, their up stream provider, the ubuntu servers, the
load on those servers..... it might get slow and it might stop. If you
have a decently fast connection and it stops, just walk away and let the
time pass to see if it picks up again.

Your ISP might have a download limit. There are lots of possibilities.
But none of those include MS being able to stop you from downloading
something.

On this page
http://www.ubuntu.com/getubuntu/download
the only problem I see on either browser is where it says, "Choose a
location near you" the drop down box spans off the screen. Its too wide.
When I click Start Download in either browser, it works.

If you have a pop-up blockers, a site blocker, NOScript, Phishing
filters in stalled, check to see if any of those are causing a problem.