This can be done in the options menu somehwere. Can't remember, but I have it set so that every new window opens in a new tab.
Earlier I too installed IE 7. started giving several problems including weblayout changing. etc. I immediately uninstalled. Now Happy with my firefox fully loaded with all the must have plugins..
I cannot open some websites with IE7 RC1 for example imageshack.us wont continue loading. im using FF now and it's a lot faster.