|Boot up to the OS that you want to keep then just delete the one you don't want. That is the Windows, Documents and Setting and Programme Files. That's assuming it is in a different partition than the other which should be the case. If by chance you put both installations in the same partition then it is not so easy. The only safe option there is a format and reinstall.|
You will then need to modify boot.ini to remove the line that relates to the deleted OS. You can do that from start-up and recovery in System Properties. Make sure the existing OS is the default.
If the OS you want to keep is the second one you installed you will find the the boot partition, the one that contains \Windws folder is in a different partiton that the system files which include NtDetect.com, Ntldr and boot,ini. They are shared files that will have been used by both instalations. DO NOT DELETE THOSE.