Afraid not... it may work for a while but there will be a lot of bugs and wrong info in the registry.
All the various updates that you install from WinUpdate as well as AV updates and so on will have different registry entries and you will not be able to merge the two registries successfully. Further, the booting problems you have now may well cause it to recur.
You can always give it a try though... when XP is fully installed, why not backup the registry, replace with the old one and see what happens. At worst it will waste only ah hours work.
PS "This time of the morning" is 1.30pm where I am :)