At least I think they can.
Through information recently leaked, we learned that Microsoft has partnered with the NSA to make Hotmail, Outlook and SkyDrive content available to the NSA without warrants.
The SkyDrive service when running on your local computer provides access to your local hard drives. For example, you can log into SkyDrive.com on the web and view the contents of your local hard drive. Presumably if you can do this, the NSA can too.
For me personally, the convenience of SkyDrive outweighs the privacy concerns because I don’t have or know anything that would interest the NSA that they don’t already have easy access to. Still it is a concern. Simply boycotting SkyDrive and using another service like DropBox isn’t a solution, since we have no way of knowing whether that is any more private than SkyDrive.
I think we need legislation.