Windows Vista

Plus Points: Very Good Security, Very flashy look

Minus Points: Slower than XP, needs at least 2GB to work smoothly, 

Some older softwares not supported, some devices are not detected

For more details visit link http://en.wikipedia.org/wiki/Windows_Vista   

Windows XP

Plus Points: Much faster than Vista with same memory specification, most of  older software supported

Minus points : unsatisfactory Security, not as flashy as Vista, etc..

http://en.wikipedia.org/wiki/Windows_XP