Using IE7 as the default browser, the following code gets me interesting results:

start www.google.nl
start www.nu.nl
start www.symbaloo.nl
start www.ad.nl

Execution will open IE, sometimes showing the four websites in four tabs, sometimes, showing three, sometimes only www.ad.nl ... I feel www.google.nl is loaded but then replaced by www.nu.nl

As a solution, I've tried starting internet explorer WITHOUT a parameter and then afterwards using start, but then I end up with the first tab being my default homepage.

The following code will get me to open several websites in seperate windows:

start /max /b call "C:\PROGRAM Files\Mozilla Firefox\firefox.exe" www.google.nl
start /max /b call "C:\Program Files\Mozilla Firefox\firefox.exe" www.nu.nl
...

But I REALLY want to open several websites in tabs, and make sure they all open! Who can explain and help me?
i had the same problem this is how you fix it:

@echo off
start iexplore www.google.nl
ping localhost -n 2 > NUL
start www.nu.nl
ping localhost -n 2 > nul
start www.symbaloo.nl
ping localhost -n 2 > nul
start www.ad.nl

the "ping localhost -n 2 > nul" is just a harmless timer so that all the pages have time to open..

i hope that helped!
Thanks kevinwhited! Works perfectly.