If I open a new window of Internet Explorer, the old windows closes. The back button on the new windows does't bring me back. The old window isn't hidden anywhere on the task bar. Task manager only shows one IE window running.

How do I fix this? thanks

I am not exactly sure why this is happening to you. On the other hand, rather than spending a lot of time figuring it out, it seems that what you want to do is use a browser with multiple tabs so that you can open more than one site simultaneously. There are a number of free browsers that are built on IE that will do this - Maxathon, Avant, Slimbrowser, and Crazy Browser.
Given that these browsers are susceptible to the same security holes as IE, I would suggest that you use one of the Gecko based engines like Firefox, Mozilla, or K-Meleon.
Finally, by paying a small fee, there is Opera.

This is my sisters computer, she wants to use IE. Yeah, I installed Firefox as a workaround. But thats all it is, a workaround.

So hopefully we can fix this problem.

I am presuming that you are using XP and IE 6.0 and that you have done all the updates. This is how Micorsoft says to repair the problem: http://support.microsoft.com/?kbid=318378. On the other hand you can issue this command at the Run command: rundll32.exe setupapi,InstallHinfSection DefaultInstall 132 %windir%\inf\ie.inf .
But, if she has a popup blocker running in the background (other than the one provided by SP2, you may have to disable it). Further, because you don't know where she has surfed and what is in her registry, you would not hurt yourself by downloading Ad-Aware, Spybot Search and Destroy or Microsofts own AntiSpyware and running them to find any rodents planted on her machine. You should do these before attempting to repair IE.
Finally, if all these fail, I would suggest downloading Crazy Browser. It is the smallest of those IE based browsers and looks and acts closest to IE but allows tabbed browsing. Once she experiences tabbed browsing she will not go back to IE.

i found this link on a google search, i have the same problem but only the other people on my computor have the problem, i am the admin, others have admin account. op is windows xp sp2. did your ever fix your problem and if so what fixed in i am losted