It'll help to know how many hard drives you wish to setup as raid 0 and if you have all required hardware to make it work, such as Raid controller, two or more hard drives and a non magnetic phillips screwdriver.
First, if you are using your motherboard's built in RAID, reboot your computer and go into the Bios. I assume you are using SATA drives so you first need to check that your SATA controller is set up to support RAID. Next check in your BIOS whether you need to individually enable raid for each SATA slot. If your OS is already installed on one drive you will want to exclude that from your RAID array.
On exiting the BIOS your system will reboot again, this time you should see a message about pressing CNTRL plus another key normally I or H but may be others or possible F10 to configure your RAID BIOS, this message only displays briefly so it may be worth looking up in your motherboard hand book what it is going to be so that you can be ready.
RAID bios setups do vary a bit but normally its fairly easy to follow so knowing which disks you want to allocate select the type of RAID you need, allocate the disks, job done.
If you are installing the OS onto the RAID now boot with your OS install media and proceed as normal, remembering you will probably need to install a RAID driver during your OS install, under XP this is always done from a floppy, I do not know how Vista and windows 7 do it.
If you are adding a RAID to an existing OS you will need to use disk manager which is in Administrative tools in Windows versions to initialise the RAID array just as if it were a normal drive.
No, no, no. RAID will work for any drives. You set up the drives in BIOS, then use them as you want. See my signature
But, you will have to reinstall Windows, if you use RAID 0 for the OS drive. Else, you'll have to copy everything to the RAID Array for RAID 1 (I think, it may do it itself, if info is on the "other" drive.)