Some of them may be semi-legitimate, depending on how you feel about bending EULA rules. OEM licenses for Windows tend to be cheeper then retail, but are supposed to only be sold with a computer, and not reused if you change systems. Some places have twisted the EULA enough to decide that as long as it's sold with hardware (IE, a $2 USB flash drive or a keyboard), that counts. Newegg for example lists an OEM copy of Windows 7 Home Premium for $94.99.

I still wouldn't trust the deals coming in via e-mail though. I just tend to avoid doing business with a company that believes unsolicited spam is the best way to get business.