Moving to Win7 from Vista on a desktop, but sods law I've got the serial key for Office Pro 2007, but no disc.
I have a Home & Student version of Office with Word, Powerpoint, and Excel so no problem there, but I currently use Outlook, and Windows Live Mail is not suitable.
Can anyone recommend a good (hopefully free) e-mail application which I can try.

Thunderbird

It's what I use (for several years now). Win 2000 through Windows 7 are supported.