Chalenge is right, although you might be able to use some of the repair functions. For a repair install the bit depth has to match. For a clean reinstall your Product Key should be valid for both 32 and 64 bit versions. I'm not 100% sure for some OEM's, though.
You didn't mention which OS you have, but here's the link for Win7:
http://www.microsoft.com/en-us/software-recovery. It may not accept OEM product keys, though, but trying is free. Another link for various Win7 versions is here:
http://46.165.222.204/Windows%207/Eng/. For more links and information, this is a good read:
http://best-windows.vlaurie.com/boot-disks.html#full. The last link also explains how to make a bootable
Repair disk which basically is an install disk without the install function. It can be used to repair a non-booting OS. For Win7 the direct link is here:
http://windows7tips.com/windows-7-backup.html. This can be created with any other computer with the same OS.
Again, not knowing about your computer, the ready built brand ones have a repair and reinstall section on the hard disk which can be accessed by a certain button at startup.
