Firstly, it'd be worth reviewing the product manuals and other information available from the Intel support web page for the D975XBX motherboard
It's important to note that there is also a different motherboard called a D975XBX2 so make sure you've identified your board correctly. What makes you think its an MBR problem causing this and what led up to the issue? Did you make backups of any valuable data (perhaps burning it onto DVDs or using an external HDD recently)? If you have no backups and valuable data is contained on those drives, it'd be worth copying their contents to an external source before doing anything fuirther. The easiest way to do this may be to connect each of the HDDs to another PC and use windows explorer to explore whats on them and copy the files you want to keep across to a temporaray folder you created to home these files. Once you've got your valuable data safe and sound (first and foremost) then you could use the Win XP Recovery Console to fix the MBR if thats required Description of the Windows XP Recovery Console for advanced users
If it's been some time since the PC received a from the ground up fresh OS installation on freshly formatted HDDs though, it'd be worth considering a fresh install from scratch as that usually makes a big improvement in perfomance on such cluttered systems. Make sure you download all the relevant drivers you'll need from Intels website onto external media before you begin (or have access to another PC to get them from).
Does this help? Let us know if you have any further questions.
I'm assuming that you are referring to a menu that is a list of operating systems that Windows is offering to load.
If so, It's nothing to do with the MBR, the board or the hardware RAID.
Windows XP uses the file boot.ini in the root of the boot drive to control the list that you re describing.
The items for the list each have a line in the [operating systems] section.
The default= line controls which entry is selected if the timeout specified in the timeout line reached (or there is only one operating system)
Hence you could delete the line referring to Windows setup and change the default entry to refer to the correct OS entry.
Boot.ini can be edited by hand in notepad or from the startup and recovery options in Windows system properties...., but if you get it wrong you could render your system unbootable. Only try this if you are 100% confident that you understand how the boot.ini file works.