hi i have winxp home,sp2,oem.can anyone tell me how far can i go upgrading or replacing parts before my oem disc will not work, or can i change h/drives,m/board etc o/k.

Your XP OEM is limited to that one computer. You can add memory, optical drives, graphics cards and PCI cards but not necessarily all at once. Changing the motherboard will need either a reactivation of XP with a different product key or a clean install again with a new key. Changing the hard drive will require a clean install unless you add it as a slave drive or mirror your system on to it.