First make sure your computer is able to run Windows 7 64 bits.
To find out if your processor is 64-bit-capable in Windows 7 or Windows Vista, do the following:
1.Open Performance Information and Tools by clicking the Start button , and then clicking Control Panel. In the search box, type Performance Information and Tools, and then, in the list of results, click Performance Information and Tools.
2.Do one of the following:
•In Windows 7, click View and print detailed performance and system information.
•In Windows Vista, click View and print details.
3.In the System section, you can see what type of operating system you're currently running under System type, and whether or not you can run a 64-bit version of Windows under 64-bit capable. (If your computer is already running a 64-bit version of Windows, you won't see the 64-bit capable listing.)
Then I suggest you make a disk image of your system disk, in case you want to roll back to the 32-bit version. I believe the Windows 7 backup utility can do this. Make sure you prepare an emergency boot disk. Instructions on the web.
Then make a backup of your documents, personal files, and important application settings currently on your system disk.
Time to think about proper disk usage. I strongly recommend using the system partition for the system and important apps only. No data, no games. I usually reduce the C: partition to 100 GB and create a D: partition on the rest of the disk, on which I put data.
Then perform a clean install of Windows 7 64-bit. There is no upgrade path from 32-bit to 64-bit.
Then install your important applications and utilities. Configure to your liking. No documents, no games yet.
Then make a disk image of your clean system (C: drive). Keep this as a baseline, in case of severe problem (such as a virus infection) you can always roll back to this clean state.
Then restore your files, games, etc, on your data partition. Move users folders (My documents, etc) to the data partition.
As an additional step, I "freeze" my C: partition with DeepFreeze from Faronics. This makes me immune to ANY (and I do mean 100%) unauthorized change to this partition, including any kind of malware.