DigitalRiver iso download legal?

I believe you have been given the answer a few times already.

"Officially", Digital River, Microsoft, and Dell will tell you not to use it.

• Digital River provides order-fullfillment for Microsoft product downloads.
• Microsoft's documentation (NOT part of the EULA) states that ONLY OEM media should be used.
• Dell, as the OEM, has their own media and is not allowed (given above) to tell you to use anything else.

"Unofficially":

• Digital River has NEVER tried to protect the downloads; from day one, a password or authenticated connection has NEVER been required.
• Microsoft has never insisted that DR protect the links/downloads and/or hide or discontinue this source of the ISO.
• Activation with the license key (Product Key) works ... period.  Activation works automatically in some cases, and works over the phone in EVERY case. 
• Above was not the case with XP, so that fact that it works should be suggestion enough that Microsoft is not fighting this and allowing it "unofficially" to continue.
• Microsoft has encouraged (or forced) OEM's to discontinue universal distribution of installation media, encouraging instead built-in recovery tools; considering most OEM's unwillingness to work with customers on this, "retail" media becomes most people's best recourse (and some, their ONLY recourse) to restore an operating system they paid for but are otherwise unable to restore/use.
• Remember that when you buy Windows, you purchase the LICENSE, NOT the software (either the code or the media (DVD), so from where the media originates is less important than the veracity of the license itself; the "retail" ISO from Digital River is indistinguishable from the "retail" ISO you would find on a DVD you purchased at the store or downloaded from a TechNet/MSDN account.

In the end, you must decide what you want to do.  You can make it harder on yourself and pursue "official" channels to restore your operating system, which will include time delays and a modest cost ($10-40).