If you cannot find "Boot Camp Services" in registry
Lot's of people have problems with the update. Me included.
This worked for me:
1) run: regedit.exe
2) search for: Boot Camp Services
the key should be: ProductName
3) change in the same path, the Language key to: 1033
4) run the installer again
In addition to these instructions, any non-US / non-english users, please be advised that registry item "Boot Camp Services" may not exist in your system. The item name is country-specific - even on english Windows.
Therefore, instead of searching for "Boot Camp Services" in Windows registry, search for "Boot Camp" only. The "Services" part is e.g. in my system "Palvelut" (finnish for 'services'.)
By searching for "Boot Camp", you will locate the correct registry item easily (notwithstanding what 'Services' is in your language) and you can change the Language key to 409 (Hex) or 1033 (decimal).
Now the BCUpdateXP.exe (V2.1) will launch correctly and do its stuff...
I have submitted this to Apple as a Boot Camp bug. Hopefully they fix this on the next release.