http://www.outlook-tips.net/archives/2006/20061228.htm
MS is not offering a solution for OE because it is working as expected,
well, except there should not be any spell checking in OE unless you use an
older version of Office. Windows does not include spell check - it comes
from an office family product (including Works, which is on most OEM
systems), so installing only Office 2007 should mean no spell check as the
spell check engine changed and OE (being much older) can't use it.
However, there is a free solution from MS - upgrade to the new Windows Live
Mail client (linked from the page at outlook-tips). It includes its own
spell check and spell checks as you type. The Live Mail client is replacing
OE and the Mail client in Vista.