One of 2 things will happen, your PC will throttle itself, so it doesn't hit the charger's limit (If the charger has a sense pin), or the charger will turn itself off if it reaches its limit*
I don't think older pavilions did the former. 65W will probably be fine, but a 90W is preferable. (I have run a 45W i7 and HD 7670M off a 65W charger before, and it was fine).
*(If you use a no-name PSU (Which you should never do), it might not turn off, and just send you shittier and shittier power.)