First you would want to get your Promise controller's drivers on a floppy diskette. Now define these two drives as a striped array in the Promise BIOS, which is going to destroy your existing WinXP installation and all your data. Once that's done, start Windows Setup from the CD-ROM, and press the F6 key when you see that first blue screen that says "Press F6 now if you have third-party drivers, etc." Setup will get itself going and then prompt for the floppy with the Promise drivers on it when it's ready. The array will appear to WinXP to be a single hard drive, and you can set up Windows just as usual.
Are you sure you want to do RAID0, though? If one drive fails, all the data is gone.