You have to do a Clean Install only.
NO other options left 4 you!
1.Back up all your files to an External Hard Disk or Other partition.
2.Insert the Windows 7 64 bit bootable DVD in your DVD drive.
3. Restart the system, go into BIOS & change the settings for Primary Boot device to CD/DVD drive, Save it restart.
4.Format the Drive with 32-bit Windows 7, Install 64 bit Win7,Activate it & Enjoy!