Windows 7 drive letters changed

I have studied and tried for 3 days, but now I need your help.  I think after the MS update on 12/25/12, the drive letters changed (during the night) and my computer was found off the next morning.  It will not boot.  I have the HDD cabled to my laptop as an external and everything seems to be fine.  I have run SpinRite on the drive, which is only a little over a month old (1.5 Tb with 3 partitions of about 500 Mb each)  It shows now as F: 100 MB, G: (what used to be D:), H: (what used to be E) and I: (what used to be C:).  I have tried he system restore to two previous working restore points, and it has failed.  There are a few more I will try.  
a.  I cannot get into Safe Mode
b.  I cannot do Regedit (which is where MS Knowledgebase 223188 says to go)
c. I have the original disk Win 7 Home Premium.
d. If I have to reformat, I have disk for Win7 Ultimate.
e. I have made a copy of UBCD 5.1.1 but I have no idea how to use it, but it does run.  
This is a computer I had made for me at Frys several years ago, so I have no manufacturer to return to.  It had Win XP until last month.  

Can I get into the registry with the HDD as an external on my laptop?  

I am dreading the copying of all my files to make sure I have them all!  

Please help!
Susan