Do you really need to ask that question? After all, this IS a Microsoft product we're talking about. :)
Well, one reason I ask that is to sort of point out the fallacy of the old M$ canard that says Mac OS or Linux are not targets of viruses because of their relative obscurity. But Windows 7 has been out for a while now as a release candidate and is thus fairly obscure—when compared to Win XP. But Windows 7 is still plagued by viruses.
Obscurity is not protection against virus infection in Windows 7, so in many ways all the old problems with Windows XP will still be problems in Windows 7.
To celebrate the launch of Windows 7 I just ordered the Snow Leopard upgrade for my iMac—only set me back $25.00 and I won’t be plagued by viruses and my 3 year old Intel iMac will run it just fine.