Center a DIV [SOLVED]

Center a DIV [SOLVED]

Hey everyone! This forum is kewl!!
Here's my thing.. ive been trying to set a div placed preloader in he middle of the screen.. MY screen even if i got a huge scroll bar on the right side of my page. This is a form so whenever some action is triggered a preloader must be shown but this isn't an easy task to perform because i can't do it with jquery and we are certainly not willing to use prototype anymore. I am not trying to place the preloader on the very middle of the page... but show it on the middle of MY screen!... i tried many things but no luck.... any hint?

Thanks in advance!

Cheers..
Jmc.