Impulse uses MD5 error checking to make sure what you download doesn't get corrupted along the way.  The way MD5 works (and this is an over simplication, I'll grant you, but it works for our purposes), is similar to taking a sentence, assigning number values to each letter (a=1, b=2, etc), and adding up the corresponding numbers to get a sum.  That's your "MD5" (though the real MD5 creates a much more unique signature than that).  Then tell someone that sentence, and have them write it down.  If they misspell something, then when they add up the digits from their setnence, it will have a different sum.

There are several reasons they could have misspelled it.  Maybe they can't hear very well (bad network cable, router).  Maybe there is someone between you and the person that has to examine the sentence to make sure there isn't a curse word before they relay it to you, and they might mispronounce something (firewalls, security software).  Maybe the person has dyslexia (bad RAM).

To try and figure out what happening, here's a few common things:

1: Try adding exceptions for Impulse to any anti-virus/firewall software you may have, temporarily disabling it if necessary.

2: Try connecting directly to your DSL/cable modem to bypass the router.

3: Try testing your system's RAM by scanning it with memtest86+

This should cover the most common causes of MD5 errors.  If you're still having trouble, give [email protected] a shout.