Elgg Installation Problems (Apache mod rewrite)

I got into problems of apache mod rewrite even when my mode rewrite was working.I was trying to install elgg on my latest version of wamp localhost server.It repeatedly said apache mod rewrite failed during installation. i cross checked with elgg v1.7 and it worked. I also tried in xamp server but of no use. Then i raplaced the new version of wamp server with the old version of wamp server and suprisingly it worked. Even on the online byethost shared account the elgg 1.6 and elgg 1.7 (i used before installing elgg 1.8) never gave apache mod rewrite error but when i am trying to install the elgg 1.8 it is giving me apache mod rewrite error.If mod rewrite was not there then how was i able to install elgg 1.6 and i.7. doesn't make sense.And in my local wamp server also i tried to switch the mod rewrite manually. (you have options to switch it on by navigation menu also) but still things did't worked as expected.