mbr error3,bootmgr is missing

Hi I have a problem dual booting Vista premium and windows 7professional. My pc had Vista on originally then I added Windows 7 ultimate rc over a year ago which I then ugraded to Windows 7 professional at the launch time (both 32bit). , then suddenly last week I got an error message while booting up that MBR Error 3 and BOOTMGR is missing.The only way I can get the computer to boot up is using an Acronis recovery cd. As I had originally installed windows 7 professional as 32 bit I thought by re installing it as a 64 bit version I would also fix the boot up problem, this did not happen. Can anyone give me some advice on how to correct this problem.