They're at it again. Once more, Microsoft (Nasdaq: MSFT) has opened up its patent cupboard and found itself a cup of flour, to which it can add water and whip up a batch of paste to slow down a rival.

A few months back, my Foolish colleague Rick Munarriz regaled you with the tale of how Microsoft bullied Amazon.com (Nasdaq: AMZN) into a cross-licensing agreement, presumably because the e-tailer trampled upon its IP rights in the course of using Linux to service its Kindle. Mr. Softie has made similar accusations, to good effect, against everyone from Hewlett-Packard (NYSE: HPQ) to Apple to Novell (Nasdaq: NOVL), receiving similar deals in each case.

Well, success breeds success, and yesterday Microsoft expanded beyond its open-source squabbles and launched its latest IP salvo at an even more significant threat: salesforce.com (NYSE: CRM). As the pioneer of "software-as-a-service," salesforce's success poses an existential threat to Microsoft and its PC-based software model. Why buy software on CD-ROM, install in on your PC, and hope it plays nice with all the other software crammed in there, when salesforce lets you buy as much software functionality as you need, when you need it, and host it all out there in the "cloud"? Not to mention that salesforce is going head to head with offerings from Microsoft's Dynamics suite that it has spent years building to compete with Oracle (Nasdaq: ORCL) and the other big boys of the customer relationship management (CRM) space.

But why?
Why? Because according to Microsoft, salesforce's services aren't just a threat to its raison d'etre. They're illegal. In its lawsuit filed yesterday, Microsoft argues salesforce is infringing nine separate patents for "mapping between logical data and physical data," "displaying a web page having an embedded menu," "stacking toolbars in a computer display" -- and half a dozen more functions, besides.

I've read the complaint Microsoft filed and you'll no doubt be shocked to learn that while it's long on accusations, it's short on details. There's little to tell us whether Microsoft has a legal leg to stand on -- but judging from its success against Amazon, HP, et al., it sounds like most targets of Mr. Softy's legal potshots have acceded to its requests, and resolved their legal differences with cross-licensing agreements. (And did I mention that Microsoft was the third biggest patent recipient last year, lagging only Samsung and IBM?)

Foolish takeaway
Suffice it to say, Microsoft's complaints haven't done much to dent the businesses it's targeted in the past, and I suspect the situation with salesforce will end similarly. So why, you may ask, does Microsoft even bother?

And I answer: Why not? If the threat of litigation serves to distract salesforce from its expansion plans, slows it down, throws it off stride -- perhaps even inclines a few potential salesforce customers to give Microsoft's own "Microsoft Dynamics" on-demand offering a try, why not give it a shot? Can't hurt. Might help.

Fool contributor Rich Smith does not own shares of any company named above. Microsoft is a Motley Fool Inside Value pick and Motley Fool Options has recommended a diagonal call position on Microsoft. salesforce.com is a Motley Fool Rule Breakers recommendation. Apple and Amazon.com are Motley Fool Stock Advisor choices. The Fool owns shares of and has written puts on Oracle. The Motley Fool has a disclosure policy.