Add/Remove says, "No Updates are Installed"...Will not Update & cannot Repair with Install Disc. (Sorry, my description is lengthy).

I have THREE issues that MAY all be linked togther......what do you think??

[I am running Windows 7 Ultimate(free upgrade from Dell w/purchase of PC)on a Dell XPS435 64 bit PC]

I upgraded from Vista approx. 3 weeks ago.

#1 - Two weeks ago, I had an issue with installing Nero/LiquidTV-TiVo to my PC...(this is also when the problem started).

To get the "Nero" thing figured out, I took the advice from someone on Nero's Forum/Blog. The Person provided a lenghty explaination on how to remove LiquidTV completely by deleting files from the Registry. . . . I really don't know what I'm doing in the Registry, but whatever I did helped my Nero issue, but I also couldn't tell you WHAT I deleted OR if I may have deleted any wrong subfolders or whatnot!.....the next time I started my PC, is when the NEW ISSUE came up.

#2 - Upon my next BOOT, Windows Update found an IMPORTANT Update:

Security Update for Microsoft XML Core Services 4.0 Service Pack 2 for x64-based Systems (KB954430)

....and it fails everytime it tries to install saying this:

Windows Update -  Failed

Code 643  Windows Update encountered an unknown error.

"Get Help With This Error" provides little/No help...but it says this:

 "WindowsUpdate_00000643" "WindowsUpdate_dt000"

From all the searches I've done, the problem seems to be with Microsoft .NET Framework....and all the solutions I found talked about, " UNINSTALL ALL .NET FRAMEWORK & REINSTALL THEM "   . . . . . . I tried, and there are no .NET Frameworks listed in Add/Remove,  and when I press "VIEW INSTALLED UPDATES"....on my PC it says, **"No Updates are Installed on this Computer".......**what the ____?

#3 - Today I figured that I'd just pop the WINDOWS 7 Install Disc in and do a Repair/Upgrade.....It won't Upgrade.

 It cycles most of the way through, then it STOPS and says this:

The following issues are preventing Windows from upgrading. Cancel the upgrade, complete each task,(it only lists ONE task), and then restart the upgrade to continue.

• Windows needs to be restarted so necessary changes to system files can be made before continuing. Restarting doesn't help. Tried it 3 times and it ends up doing the same thing.

Is it possible that I deleted something out of the Registry that day and that is what is causing all of this?

 If so, is there a way to fix it WITHOUT doing a fresh install of Windows 7??

OR.......whatever else may be wrong....PLEASE HELP! 

(And please note: I am a college freshman majoring in Computer Science...but I just started last semester, so I don't know everything I should...yet. - Although, I SHOULD'VE known better than to mess with the Registry!! My bad.)

Thanks so much for your time, in advance, and I look forward to hearing from you!  VikeMike4