I have had this happen three times now..
I buy a barebone sydtem put it together and I put in the old hard drive from my prvius computer with windows already installed.. When I fire up the new computer I cant get windows to boot from the hard drive any more so I just reinstall windows and then it works fine. But now I upgraded my wifes computer new everything except the hard drive...I can just install windows again but there are some old programs that she need for work so I need to try to make the computer boot with the old windows.. any ideas..

PS
I tried to do a repair of windows from the install cd but its asking for an admin password... this is an old computer I have no Idea what the admin password is..

Download Offline NT Password & Registry Editor and change the admin password then do a repair install, it should then update the drivers and should leave you with a perfectly bootable operating system.

Make sure you read the FAQ's on that page though.

Specifically this one:
Hope this helps.

thanks lebowski,

I will try that tonight

I would like someone to explain why the motherboard doesnt recognize that there is windows on the hard drive and boot to it

Mainly because of drivers i imagine.

After you set the machine up it expects it to meet that profile and is no longer using generic drivers like it does during the setup so that's probably why.

This may not be 100% correct, but I'm guessing so XD

that makes sense to me...

but I am always hearing of people totaly upgrading their systems without having to reinstall windows.. which wipes out all your programs..