I had the same issue when my Dell Vostro arrived with Office 2007. However, following recommendations (after getting completely hacked-off with the rubbish known as Windows Live Mail), I contacted this chap in Oz:
Outlook Express Email, Fix Outlook Express 6 spelling, Microsoft Outlook Express emails/mail, Spell check for Outlook Express (spellchecker, OE, spell outlook express, outlook express spell check, outlook express vista, outlook express 2007, outlook
A quick overnight PayPal for Au$ £10, then first thing the next day I followed the links and voila, English spellcheck returned!
As I wrote back to Kelvin:
"Hi Kelvin, Excellent - thank you so much. The program does just what it says on the tin! Great to have OE6 back again instead of the bug-ridden Windows Live Mail! I'll pass on details of how to solve the Office 2007 / OE6 spell check bug to other folks. Once again, thank you for doing what Bill Gates can't be bothered to do! Best wishes - From Great Britain"