Google made a splash late yesterday with the announcement of its upcoming Chrome OS, a lightweight and web-centric operating system based on Linux with which they hope to conquer the netbook market. But while the move has been received with both skepticism and hope for a real Windows challenger, some believe the search giant’s timing was merely an attempt to steal Microsoft’s thunder. Apparently, the Redmond-based company is waiting to make a big announcement of its own next week.

Tech blogger and former Microsoft employee Robert Scoble has posted a message on his FriendFeed account today, where he notes: “Why did Google announce Chrome OS this week? Well, of course, Microsoft has a big announcement coming on Monday (I'm embargoed).” Intriguing… so what could Microsoft have in store?

Neowin reportedly "confirmed" last week that Microsoft will announce Windows 7 has hit RTM on Monday at the Worldwide Partner Conference that is taking place in New Orleans. But that doesn’t sound like much of a big announcement, so speculation currently centers on a re-architected web browser known as “Gazelle,” which acts like a self-contained operating system and is designed to support web applications in a secure manner. The company has been dropping hints about the new browser, but it is unclear how close it actually is to release.

Another – perhaps more likely – idea being thrown around points to a public rollout of Microsoft’s Office Web-based software, which is expected to include lightweight versions of Word, Excel, PowerPoint and OneNote. According to reports, it'll be delivered to consumers through Office Live, in both ad-funded and subscription-based versions. Whatever the case, it seems Microsoft and Google are set for a clash on Monday.