Microsoft (Nasdaq: MSFT) SharePoint has a big target on its back. The company setting its sights on Mr. Softy is Silicon-Valley startup Box.net.

Privately held Box has been gaining traction with business users and IT departments all over the place. The company has garnered 7 million users and 100,000 businesses and tallies up 250,000 new users every month. Its cloud-storage and collaboration platform competes directly with SharePoint, which commands 100 million customers. Although it's not quite a "SharePoint killer," the growing company represents a threat that Microsoft can't ignore.

Box has just secured $81 million in funding from new investors, including salesforce.com (NYSE: CRM), valuing the company at more than $600 million. Aaron Levie, Box's 26-year-old CEO, plans on using the cash to focus on infrastructure, build out a third data center, and invest in R&D.

This comes shortly after rejecting an unsolicited buyout offer from cloud heavyweight Citrix Systems (Nasdaq: CTXS). Box is getting cozier with Google (Nasdaq: GOOG), which has had its hands full lately and hasn't been focusing on enterprise. Box would be a no-brainer acquisition for Big G if Levie weren't insistent on keeping the company independent.

In a direct shot at Microsoft CEO Steve Ballmer, Levie recently took to the stage at the company's inaugural customer conference, Boxworks, and said, "Enterprise software isn't sexy, and that's because it makes you think of this guy" -- as he displayed a picture of Ballmer. Box is also going after other enterprise-software players, including IBM (NYSE: IBM) and Oracle (Nasdaq: ORCL).

Last year, Microsoft announced Office 365, a major upgrade to its staple Office offering that promises to make the software much more cloud-friendly. Microsoft initially launched a beta period and then pegged next year for widespread availability.

As much as Levie resists offers, this Fool thinks he will eventually give in and sell the company to a bigger player. In the meantime, he'll be keeping Microsoft on its toes playing defense.

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