indication error with opera and ie

dear developer(s),

a friend just told me, that the right sidebar of my new kubrick-themed drupal (4.7.0)-site looks strange with opera, so i checked that out and indeed. with opera 8.51 and 9.01 for win32, the right sidebar blocks slip into the content-column. the login-form, the chatbox and who's online information can't be seen at all: screenshot opera 9.01.

i checked out ie 6.0, too, and the headlines and normal paragraphs of the right sidebar blocks are centered. though everything can be seen, it looks a little strange, too: screenshot ie 6.0. with firefox 1.5.0.6 everything looks just fine: screenshot firefox 1.5.0.6. by the way, the original wordpress kubrick-theme hast no problems with ie and opera.

i changed some things in the css. as you can see, i took another header-image and redded the anchors. i changed the lenght of the login-form and the look of the forum-tables by adding some code, but i commented those out and it didn't made a difference in the indication error i've just described. though most of my visitors use firefox, i'm not too glad with those bugs, because second most users come with ie and 11th most with opera. do you have any ideas, how to bypass those problems?
--LeV