Disk 1:
c: Windows XP
d: Storage
Disk 2
g: Storage
Disk 3
f: Win 7 64bit
i: Storage
The easiest way is when you have a new drive that has nothing on it. Win7 installation process allows you to pick a drive and partition to install to. As windows 7 prefers to play big brother it automatically boots this OS first but as part of the boot up process you are offered a menu asking which you would like to boot. You can change this with a free program called EasyBCD.
You've got a couple of options:
1:
Use your smaller drive for both OS's and move your data to the big drive.
2:
Use your big drive for Win 7.
The esaiest is option 2 if your big drive data can be moved over to the other drive. Partition it and give Win 7 about 60gb space and partition the rest as another drive for more storage.
Run the windows 7 installation and select custom and choose the big drive 60gb partition to install to. I don't see much point in giving an OS more than 60gb. It makes it a nice size to work with and means backing it up is a lot easier to.
I'd also go for moving your system virtual file too. Put the XP page file on the big storage partition and Windows 7 virtual file on the smaller drives large partition. I know it sounds a bit much but it reduces disk activity if its ever used. It would probably be best to explain it over steam chat using showmypc so I can show you some screen shots of what it would be like. I could also pre-partition you drives so the installation should be a little easier.
The last thing to do is make sure XP and Win7 use the same steam installation. If you installed steam to c:\program files\steam (default location) then you can move the steam folder to your large partition on your large drive. Delete all the files in steam (DO NOT DELETE steam.exe or the FOLDERS - those are your game files). Double click Steam.exe in XP and let it reinstall itself. Reboot and run Win7 and delete all the files except steam.exe and the folders from the steam installation folder and rerun steam.exe. That way you can use one steam installation across both operating systems.
Some one needs to confirm the steam thing as I've done it once but it just needs checking
