Hi
You may have done this already. But you first have to copy all the data from your profile folder which is inside the users folder on the c drive onto the D drive as this is where all your documents, photos, email (Live Mail, Outlook etc) and such should be.
Reboot the PC and insert the Windows 7 install disk.
you would let it run and when given the option choose a custom/advanced install.
Then select to format the C drive.
Once the install completes. Copy only the documents, desktop, photos, videos folders from the backup on the d drive into your new profile folder in the users folder on the c drive.
Reload any programs you require.