Access to Win 7 is gone after WinXP installation

The computer originally had a 300gig Velociraptor with Win7 on it, and a 1 terabyte Hitachi for storage. I added a 150gig Raptor and set the boot order in BIOS to boot from the CD drive. I installed Win XP onto the 150gig Raptor.
I then set the boot order in BIOS back to harddrive, making sure that the 300gig was 1st, the 150gig second, and the 1 terabyte 3rd.
If I just let the computer boot up, it goes to XP.
I can interrupt the POST and access the boot menu during POST by pressing ESC.
However, no matter which drive I select, the 300, or the 150, the computer boots into XP which resides on the 150.
When I get into XP, looking under MY COMPUTER, it shows 4 harddrives, not 3
They are called SYSTEM RESERVED(C: ) and is 99.9 megabytes big. The next drive is called NEW VOLUME (D: ) and is the 1 terabyte Hitachi. The next drive is called LOCAL DISK(E: ) and is the 150gig Raptor, and the next drive is also called LOCAL DISK(F: ) and is the 300 gig Velociraptor that contains Win 7, I hope.
The numbers aren't exact, as you probably know that windows always shows a drive capacity at some number smaller than advertised. I have no idea where that ~100megabyte drive C came from, and how it shows approximately 40 megs used space. I suspect that it "borrowed" the 100megs of space from one or several of the other drives(?).
Is it possible to regain booting ability on the 300gig drive and Win7?

 

Try a repair of the Windows 7 drive. When installing an earlier OS after a later one, it messes up the MBR. Doing a repair should correct it.

 

Wow, that was a fast reply!
Don't let my rating as Senior Member with Intermediate experience fool you.
I am evidently a novice, and would be dead in the water without the nice folks here on WindowsBBS.
Could you tell me exactly how to do this repair of the Win7 installation?
Do I boot from a CD drive and direct it to the 300gig HD?
Thanks.

 

Yes. You will need to have your windows 7 CD and boot from the CD. Here is a walk through.
You can also edit the MBR following the instructions found here.

 

Sorry for the delay in responding, Crunchie.
Both the walkthrough(first link) and repair of the MBR(second link) start from within the operating system.
The crux of my problem is that I cannot access Win7 on the 300gig HD.
I can boot into XP on the 150gigHD. Am I misunderstanding the instructions?
If I can't get into Win7, then how do I execute the instructions?
I know that I can access the 300gig HD from within XP either through Windows Explorer or My Computer. Can any of those steps be accomplished that way?
This is aEVGA X58 SLI LE motherboard. It has 6 SATA channels. By hitting ESC during POST, I get to the boot preference menu. SATA0 is the 300gig, SATA1 is the 1Terabyte Hitachi, and SATA2 is the 150gig with XP on it. 3 is empty and 4 and 5 are DVD drives.
As I mentioned, within WinXP, there are now 4 harddrives shown-the C drive is the one that I guess has been virtually created-if that is the correct word-because I do not have a 100megabyte drive installed.
Would booting from the DVD drive with the Win7 DVD installed permit me to run setup again and this time run repair, or is that not the way to go? I've attempted that before in the distant past when I tried repairing XP, but I recall that it asked me for the administrator password, which is something I never established when I first installed the OS to begin with. I always gave up the repair effort and ultimately formatted and reinstalled every time.
I read through the explanation of how earlier Operating systems overwrite the previous MBR, but I cannot comprehend where that MBR resides. Is it on the 300gig drive with Win7 or on the 150gig drive with XP?
Also, what of the 100meg drive that is called "system reserved "?
Thanks for your continued attention. I hope I've demonstrated that my Senior Member status is just a function of time passed as a member, not based on any sort of knowledge level.

 

With the repair step, you do not have to go into windows. That was only to disable an anti-virus. Boot straight from a CD and go from there.
You van also use EasyBCD to restore the dual boot. Have a look here and follow the instructions.
Just replace Vista for 7 .