I pulled up your original post--let me first address some of the questions there and then we'll get to this.
1. How many partitions? You got some pretty good answers; Red Hat will insist on at least a / partition and a swap partition. Beyond that is up to you. For right now, let Red Hat set it up as it wants.
2. Both Windows and Linux will fit comfortably in 5 GB partitions; anything beyond that is gravy.
3. Boot disk required. The two warnings are loosely connected. The "constraints for your architecture," you don't need to worry about. This refers to computer that are mostly over six years old. The BIOS of those computers wouldn't boot anything beyond cyliner 1024--you'll hear this referred to as the 1024 limit. Modern computers don't have these limitations. If your BIOS can handle XP and a 120 GB hard drive, this warning you can ignore.
What you cannot ignore is making a boot disk. Alway have a current boot disk for each operating system--you will need them.
4. Error 28. In you Linux documentation, you will find instruction on how to access the other consoles during problems. What was causing this error would have been there.
Okay, where you are at today. You can only boot Red Hat from a disk and it just goes to a Command Line Interface (CLI).
Your problem is probably your Nvidia GeForce card. Read this:
Best bet is to reinstall Red Hat and tell it you have a GeForce 2 card.
For the boot loader, Red Hat's default is to put Grub in the Master Boot Record. This is the easiest thing to do and it usually works. To make sure that Grub recognizes your XP partition, you need to name that partition when you are in Disk Druid--select it and make sure it says "Windows XP" or something like that. You will also see it when Grub is asking which should be the default operating system.
If Grub doesn't install, try LILO in the MBR.
The reason you aren't able to boot into Linux from your hard drive is probably because you put the boot loader at the beginning of the Linux partition. It only goes there if you are going to use the Windows boot loader or boot from a floppy.
Finally, here is a nice fact sheet from Red Hat on dual booting: