The homepage on the following link shows with a white background in Firefox, but in Explorer almost all the white gets transparent. What should i do to get it to work with explorer (and netscape)?
http://www.hallevikslagret.se/Hemsidan/index.html

That's not your only problem.
The tabs that you have at the top barely show in Firefox, it's like they're way beyond the top of the browser window and you can't access them. The Iframe isn't working -- and they're a bad idea anyway.

You're also telling it this:
style="FILTER: chroma(color=#FFFFFF)" allowtransparency="true" So, it appears to be doing what you've told it to do.
All that, by the way, is proprietary and will cause you cross-browser troubles.

You also should not do this:

html {
margin: 0;
padding: 0;
height: 100%;
overflow: auto;
background: no-repeat;
}

It's not appropriate and not necessary. Create yourself a container <div> and set those properties on the container. That code is also the reason why the tabs disappear out of reach in FF.

You should also dump the tables and put in a DOCTYPE of at Least HTML 4.01 Strict.

I took out the "allow transparency" and all the styles on html and the white shows up fine in IE 6.

Neither the html or the css information on your site validates either. Add a doctype and specify the character encoding method then we can see exactly what the validator believes you're doing wrong. The site looks really good in firefox, it just needs that extra little push to become brilliant.