PC freezes, then refuses to boot (bootmgr is missing)

I just turned on my PC and everything looked fine, but when I tried clicking an icon it turned out the desktop was frozen. I tried bringing up task manager but it didn't respond either. About 5 mins later the screen went black but I had a movable mouse cursor, about another 2 mins later an error message popped up saying something like:-

"could not load log-on controls to open task manager. Press escape to restart or the power button to shut down"

Neither option had any effect so I forced a shutdown. When I turned the PC back on, as soon as my 3 SATA HDD's had been detected, it stopped booting and said "bootmgr is missing". I don't understand what could be causing this, I haven't changed any deep advanced system settings or made any BIOS changes since I installed Win7 a couple of days ago.

One thing that struck me as strange from the start though, was that the first couple of bootups after the switch from XP-Win7 gave me the option to boot either 7, or an unspecified " earlier operating system ", but this choice only appeared twice. I think what caused this was that I forgot to format my old 20gb IDE system drive before installing 7 on my newer 500gb one to make it the new system drive. Still, if this is the cause, why would it wait two days before messing up? I tried disconnecting the old drive but it made no difference. I also checked the boot order and everything seemed normal. BTW I originally booted from USB to install Win7 but I disabled USB booting afterwards. I burned a recovery disc which might help, maybe all I need to do is format the old empty xp drive which I'm guessing would be simple if the disc allows me to boot, but again everything was fine for 2 days so I'm not sure if that's the cause.

Damn, just when you think you're in the clear... Does anybody know what might be causing this? I could really use some help!

Thanks!

 

Edit: After reconnecting my old XP drive a couple of times, then choosing to boot from it, I got the dual boot menu again and chose " Earlier OS" and now I'm booted into XP. So when I installed 7, I accidentally created a dual boot system (lol) because of my formatting mishap. Hopefully all I'll need to do now is simply format the 20gb IDE and remove it. Stupid question, but formatting the drive I'm booted into is suicide right? I take it I'll need to boot into Win7 (if it let's me) before I do that?

Cheers!

 

I tried booting into Win7 again (although I had to boot from the old XP drive to give me this option) but if I try to open task manager or network connections, the system freezes for about 5 minutes then goes black as mentioned earlier. However, this time I pressed ESC and got back to a working desktop. I assume Win7 Taskman & network connections button can't be loaded because somehow I've booted into Win7 using XP (as if both OS are running at once, fighting each other). For some reason I just can't boot Win7 from its own drive anymore. Somehow I need to format my XP drive but thats probably not possible unless im booted into Win7 using the Win7 drive. I'm at a loss...

 

Sorry I keep missing the 'edit post' timeframe so I have to make a new post, I'm not just trying to bump!

Anyway, strangely enough, while booted into XP, I can still load all the programs that were installed on Win7, even 64 bit programs on a 32bit OS (WTF?). Does this mean technically these programs were installed with XP and have their reg entries in XP's registry? If so, how did they work on Win7? What on earth is going ON here!?