I'm a tinkerer, not a coder, so there's probably a better answer than this, but one way might be to find this bit of code in style.css
/*
One-column structure for archive and search pages
*/
.search #container,
.archive #container {
float:none;
margin:0 auto;
width:940px;
}
.search #content,
.archive #content {
margin:0;
width:940px;
}
.search .main-aside,
.archive .main-aside {
display:none;
}
and change the widths to match the home page, the settings of which are just above that in style.css. Remove the "display:none" to get the sidebar back on search and archive pages. This is what I did, but for some reason the sidebar is still not showing on search results page, so I'm still trying to figure that out. (Help, anyone?)
I'm sure Ian would say "it's best to do this in a variant". (Search for 'variant' in the forums and you'll see what I mean - it's pretty easy to set up.)
Good luck.