When you say Installing "After" do you mean as an upgrade from 4029 to 4051 or as above suggests a seperate install??
If you wish to install Longhorn 4051 on a system with 4029 already installed you will have to create a seperat partition (Min 3GB) for 4051 to be installed.
Once installed you will have to edit the "boot.ini" file the root/active partition and it will dual boot just like any other two OS's
I would not recomend upgrading to 4051 from 4029 as the PDC versions which support upgrade seem to have more faults than a clean install.
Vaughaag