Google Checkout arrives today, and with it yet another Google service that may or may not lead to increased Internet/world domination. It's the dominant search engine, the primary provider of text ads, and one of the more popular email services around. Will Checkout take down the controversial yet ubiquitous Paypal? Many have tried but all have failed so far.

Meanwhile, Google's got a number of other fights on its hands. Some are in full pitch, some are merely brewing. To wit, here are the five biggest fights on Google's hands. Can Google really handle this many battles on so many different fronts, all at once? You be the judge.

Google's Five Biggest Fights

5. Google vs. Yahoo! - Easily the company's oldest rivalry. Google and Yahoo! offer so many similar services (search, ad services, mail, calendar, IM, news, maps, shopping, you name it) that you can't spend much time on the Internet without using offerings from both of them. I will abstain from commenting much on this fight, since Yahoo! pays the bills and all, but it's the primary battle for both companies on a day to day basis, and one which will probably stretch on for years.

4. Google vs. Apple - Stick with me and you'll see why. On the surface, Google and Apple have very little in common, but that could change soon. The maligned Google Video is probably just a precursor to a full-blown movies-and-music shopping site, which would compete directly with iTunes, the bread and butter for Apple today. And the long-rumored Google PC could be a direct competitor with the Mac Mini, should it ever come to pass.

3. Google vs. Microsoft - Another war that's been in the making for a while and is finally starting to heat up. The battle vs. Microsoft is actually much more mature than the one vs. Apple, and here's why: You can do almost anything on Google now that you could do with a Microsoft app: Word processing, spreadsheets, and even image editing are now Google features, and they're free. Still, while Google's Picasa wins raves, its office apps have a long way to go to reach maturity, and consumers may never be happy with the idea of storing files in the ether instead of on a local PC. A bigger question: Could this be the beginning of a Google-branded operating system, which could solve all these problems and take Google against Redmond head-on? Rumors have been flying for months.

2. Google vs. eBay - With Google Checkout this battle begins in earnest. Google Base is a rudimentary commerce system, and Checkout now adds a simple payment scheme. Will this impact the auction monopoly of eBay and the payment monopoly of Paypal (part of eBay, if you didn't know)? It might, thanks to a trick up Google's sleeve: You get your Checkout fees waived if you spend enough money on Google AdWords. That's a great business trick that hits eBay exactly where its users are the angriest: The ever-increasing fee structure that a greedy eBay hits auctioneers with on a quarterly basis. Another point: A serious number of Google ads and Google search results currently point to eBay auctions. If those redirect toward Google services, eBay could lose a good chunk of business that the search engine currently provides it. A mass exodus may not be in the cards, but I wouldn't be surprised to see Google start chipping away at this powerhouse.

1. Google vs. itself - Microsoft used to be unstoppable, too, but then corporate bloat got the better of it, the Department of Justice cried foul, and users got tired of buggy, half-baked software. All three of these issues could be the downfall of Google. Managing a company of 1,000 is a much different task than managing one of 10,000, and as Google grows, its quickness will undoubtedly be affected. As middle management takes hold (and it invariably will), Google could run afoul of the same problem that has led to years-long delays in virtually every Microsoft product release. Monopoly is another biggie. Once the DoJ starts sniffing around, it's pretty much over. When "Don't be evil" starts having to pay fines for anti-competitive behavior, sell your stock. Then there's the bugginess issue. Well, sadly, this is already upon us to some extent. Google Base and Google Video have been flops (so far), and Gmail still can't sort your mail by subject line. I don't really like Google Desktop, either. Is this a sign that the innovation well is drying up at Google? Or that it's just rushing services to market without much forethought? Google's fight against its own complacency will undoubtedly be its biggest challenge ever.

Disclosure: Yahoo! Inc owns and operates Yahoo! Tech. Opinions expressed by the Advisors are their own and do not necessarily reflect the views of Yahoo! Inc.