By corrupted screen, im assuming you are refering to rather random output that is in textmode.(Random blue-green-gray colors with text mode symbols.) Well, in any case... I'm guessing its a driver related problem. First things first: try to kill the xserver. This is what i guess is ctrl+alt+backspace.
Now you should be in textmode. You may... rather probably need to log in threw there. Best you log in as root. Personally... i've never used RedHat. Ever. So, youll have to find out for yourself what you need to do to configure the X Server correctly. What kind of card do you use? you may need an external kernel module. I've got a nvidia card. Tho i've never had a problem with the opensourced drivers, they do not have 3d acceleration. so i downloaded nvidias and compiled it. It requires kernel source, and you MAY have to point it to the correct path to find the kernel sources. i did running SuSE Linux 10.0 and the kernel sources from YaST. If you have another card please look for instructions on installing there drivers. If you require a graphical interface to do so, in the command line, you may be able to find xorg.conf and modify it to do so. You know what? i can't tell you anymore cuz im a n00b.
Now to the windows XP problem. If restoring the boot loader worked, somethings gone wrong when redhat installed a bootloader! I dont use windows whatsoever.. but my brother has used linux with GRUB next to windows XP home ed. No probs there. If your running lilo, You might as well switch -- grub doesnt have to be configured as much. BTW... If you can, try to use the redhat installcd to boot your RH system. its still installed, of course, but rather unreachable because windows will basically without warning overwrite the bootloader of anything else without giving paths to boot from it (Hell, even ReactOS will do THAT!).
Also, you are worried abouth 64bit not working? Absolutly not. 64bit mode will work with a working 64 bit computer. its your platform. Dont be afraid to try to make it work. Good luck