linux replaces windows completely.
(there's various more advanced setup methods, one where linux runs inside a virtual machine
, one where you have 2 seperate partitions
on your hard drive -one with winxp, other with linux)
...but for all intents and purposes,
linux replaces windows.
they are both operating systems
, and not programs. only one operating system can run at a time on one computer.
for your other questions, the answer is:
most probably yes, but it depends on your hardware.
send us some specs, you'll get some more answers.