Hi newcomer and welcome to the Tom's hardware forum.
1- Why u need clear the memory? Look for the button or jumper in the mobo and real the manual to know the process for clear the memory.
2- If u want have the options to select the OS that u want use, select run (windows key + r) write msconfig, boot tab and check if u can see both OS XP and Vista and the timeout should be twenty or more.
3- If u can't see the two OS in the boot tab download this program install it and u can add the 2nd OS to the boot options.