Its along story,as to why i want do this.im having problems.

Windows 2000 is an operating system and yes you can install it. However you will loose XP, unless you install 2000 on a seperate partition and dull boot.

Typically you should install operating systems in the order of oldest to newest, if you want to dual-boot that is. If you want to install 2000 when you already have XP installed, format your hard drive and then install 2000.