^^
There will be 2 boot loaders: one of Vista and one of XP.
U'll hv to decrease time-outs of both boot-loaders. So just do following in both OS:
Open "sysdm.cpl" and goto "Advanced" tab and click on "Settings" button in "Startup and Recovery" section. Now u can decrease the time of "Time to display list of operating systems".
U can also do the same by using "msconfig".
