Keep a close eye on Cisco Systems this week. The networking giant kicks off its annual partner summit April 2 in Las Vegas. In addition to taking on the usual suspects (Avaya, Nortel, etc.), Cisco will spend the week targeting Microsoft, The VAR Guy hears.

During the conference, Cisco executives will describe how the company will interoperate with Microsoft as well as compete aggressively against the software giant, according to those in the know. Also, Cisco CEO John Chambers will take the stage on April 5, rallying partners to cash in on unified communications.

For those still getting up to speed, unified communications builds upon the current voice-over-IP (VoIP) craze. The technology provides users with a single in-box for all of their email and voice mail systems. Moreover, unified networks will automatically route phone calls, emails and other types of messages to the most appropriate location–whether you’re in the office, at home or on the road.

Although Cisco is working a bit with Microsoft on unified communications, Microsoft is more closely aligned with Nortel. It’s a classic Microsoft strategy: In order to sink a market leader (in this case, Cisco), Microsoft typically partners with the market leader’s chief rival (in this example, Nortel).

Cisco and Microsoft have been marching toward a network software showdown for more than a decade. During the 1990s, the two companies partnered to port Microsoft’s Active Directory to the Unix operating system. But that effort faced multiple delays and failed to fulfill its early promise.

More recently, Microsoft entered the low-end networking market and briefly competed against Cisco’s Linksys division. But faced with tepid sales and fierce competition, Microsoft killed its WiFi networking products in 2004.

These days, Microsoft is striving to ensure that Windows Vista and the company’s server software support Voice-over-IP (VoIP) phone systems. On the flip side, Cisco is developing its own Windows-based telephony applications, which connect to VoIP networks. It all adds up to the biggest showdown in the IT industry.

The Cisco vs. Microsoft battle won’t end this week. Both companies will update their unified communication strategies in May during Interop, a major networking show in Las Vegas.