yes u can install another windows at the side of the old installation if u like and the old windows is renamed windows.old
,
this takes up more of your harddrive
if u want to start from complete scratch
back up all your files on an external drive
insert the cd and boot from disc
part of the insrtallation process says about the location where you want windows to be and there is a button that says advanced.
click this and then click on the main drive
click the little pink rubber that says format
this will wipe all files off your drvice then you can do a fresh install
by the way, bugcheck 0x0000007b is inaccessable boot disk and can be a pain to fix, so you prob did the best thing