May I just add on the subject of the 'wndows.old' folder; if you had already backed up your previous install (to Removeable Disk, or separate drive) and especially if you have used a 'dual-boot' configuration, you can 'delete' the windows.old folder and recover 20Gb of space.
Useful if you wish to make use of the space for other files
Indeed, very true, and also useful if you want to copy your Windows on the same harddrive when normally some windows files are write or copy protected. Nahh, don't do that unless you really have to or a pro! Because the reversing process might be a bit harder to do with 1 and only main harddrive.
Anyways, also I need to correct Microsoft ONCE AGAIN!
Installation of Windows 7 doesn't require 16gb, but less than 9gb actually!
That's almost half of what they say we'll need as MINIMUM. Total non-sense!
Don't believe everything that your read, but keep an open mind though!