As Han up here said, you need to validate.
If you don't validate, IE will display your page in quirks mode. Which means it'll be screwed up. If, after validation, you still have issues, you'll need to check the width of things in your sidebar and content against the actual width of the sidebar and content divs. If the stuff *inside* is bigger than the divs themselves, it's gonna move your sidebar down. (There are *tons* of threads on this *exact* issue here. At least 4 a day get posted here, and almost always given the same answer.)
The widths, by the way, also include your margins and padding. So if your div width is, say, 200px wide, with no padding or margins, but the image you've placed in therer is 190px wide with 10px padding, you're gonna be too big (190px +10px on left and 10px on right="210px. Too big.)
Long strings - such as long URLS or something will also cause this problem in IE.
So validate first, then if you still have the problem, start looking for "too wide" things.