I have an old pentium 4(2.67GHz) pc with 1760MB RAM, 80GB Hard-disk. I was wondering if i can install something different than windows xp on that.
It's quite possible to run Mac OS X Mountain Lion without much of a lag, if you install an aftermarket dedicated GPU. This is because Mac OS X works way better with the Intel core processors because they have integrated GPU. So having an aftermarket dedicated graphics card will let you run Mac OS X. Also check if your motherboard is supported or not. Let me also remind you again that what you are about to do is illegal.
Apple's OS X is only able to run on Apple hardware. In small part because of driver limitations in the software due to Apple's limited hardware ecosystem that they need to support but largely it's due to the fact that Apple's licensing for OS X forbids running the OS on non-Apple hardware.
This topic has been well-covered on the site. For similar questions and answers please see:
- Is there a way to evaluate OS X without purchasing Macintosh hardware?
- Can I install Mac OS X Lion (10.7) on my Windows XP PC?
- Is installing Mavericks on Hackintosh legal?
Question regarding how to install OS X on non-Apple hardware are off topic here as they violate the EULA.
None, as @IanC points out. Try running a Linux OS like CentOS or Ubuntu...
But if you feeling adventurous and start down the OSX86 (aka Hackintosh) route, be warned that you'll spend time tuning, tweaking and otherwise managing your install. If you're the type that enjoys that work then you're in for a ride. But if you just want your computer to work for you, stick with Windows or a Linux variant.