Hi. Although I dont have a mac, but I'm just curious to know if it is possible to run windows 7 or Windows Vista on Mac
(I've heard of windows XP and OS X Leopard running together... So I thought windows 7 or vista might also be possible!)

You can just use either BootCamp, which comes with Leopard, or purchase a separate program like "VMWare Fusion" or "Parallels". I prefer to use Parallels since you can just run things in a window and it's very convenient.

Just installing Windows on a platform like an Intel-based Mac is quite easy but you need to keep the following in mind:
For doing a Dual-Boot:
You will need the Mac OS X version 10.5 "Leopard" operating system installed to run Microsoft Windows. Should you have an operating system lower then version 10.5 you cannot run Microsoft Windows either not very well or it's not even possible.
This is because version 10.4 was the first to support the Apple-Intel architecture with support for Unified EFI (UEFI) without any support for the older system BIOS standard used on all Intel IBM-compatible PCs that have come out since 1981.
Windows Vista 64-bit SP1 is the first Microsoft Windows operating system to support UEFI and is a big part of the reason for replacing the older NT Boot Load (NTLDR) with more flexible Windows Boot Manager (bootmgr) system with Windows Vista RTM.
Due to unavoidable delays* the support was not finished in Windows Vista SP1 64-bit edition to allow booting Windows from the hard drive with hardware supporting EUFI.
The support should be in Windows 7 and I would assume Windows Vista SP2 but data on that is hard to find because Microsoft is very silent on released information about what Windows Vista SP2 and Windows 7 will support.
OS X 10.5 updates the Mac computer's firmware to include a very small subset of support for system BIOS technology to allow any Operating System that supports system BIOS technology to boot on a Mac.
For the sake of marketing Apple just called this support "Boot Camp" as otherwise calling it "BIOS system compatibility for UEFI on the Intel-Mac arch plus a Graphic User Interface to make using it very easy" is both way to long and it wouldn't make any sinse to the average user who wants to run Microsoft Windows on their Intel-Mac.
There are many more minor issues that OS X solves with it's Boot Camp software that I don't even have time to get into.
The short of it is be sure to have at least OS X 10.5 installed and all the recent updates for Boot Camp installed or you could easily run into "Dual-Boot Hell" which you are really just Dueling Windows attempting to install and run it :D

For Virtual Methods like "VMWare Fusion" or "Parallels" they work quite well but you need to keep the following in mind:
1. They require a Intel Mac with fairly good processer and a good amount of RAM.
Any Intel Mac you buy today will meet that description but some models like the current Mac Mini will run those programs slower then you may want.
2. They both have a good sized cost to them:
They each cost $79.99 but they have 30-day trials you can download and I can say from using the VMware and Parrellels products for Microsoft Windows that it is so much nicer then dual booting that it's worth the relativly small price to avoid all the problems the dual-booting can bring but if you only need to run Windows every once in a while and you have OS X 10.5 or higher with the latest version of Boot Camp then you should go for it as from what I have read, it works quite well for the times when you wouldn't mind rebooting into Windows to run a Windows only program.

I concur - all the articles Ross wrote are excellent =)