There are skins available to make XP look like Mac OSX. You'll need either Microsoft's Plus for XP or third party software like WindowBlinds or StyleXP to skin XP, though.

As far as running a Mac OS on a PC, kjanx is correct. PCs and Macs are different at their very core.

There are Mac emulators for PCs but they tend to run extremely slowly and very few who have tried them are happy with the results.