Upgrade pc from single core to dual core - same hdd

Will this go smooth with no problems?

So If i change cpu and mobo, go to boot on current hdd, it should boot up?

You made no mention of a mobo swap in the first post. As Edklite said, a repair install is in order if you swap the mobo. I know pscowboy has a novel method for this as well, but it's too complicated for me to remember. You might want to PM him about transfering windows XP without reinstalling/repairing, and he'll more than likely be glad to help.