How do I reboot a gateway that has windows xp on it

A Reboot simply means powering off and back on. It will not do anything to get rid of malware. I think you are thinking of a Recovery. This is using the Recovery CD or partition that was supplied with your system to restore the system to the way it came from the factory. This will wipe out anything installed to or saved on the PC since.

How to do that is different for each brand and model. Check your manual for specifics.
What if I don't have the cd

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

You need to get one from Gateway or look for a System Partition. When you boot up there should be some instructions like press F2 for Setup, F10 for Recovery etc. If so, press the appropriate key and follow instructions.