A while back my dad's friend put a second harddrive into the family computer and put xp pro on it. The newer harddrive is used by dad and the other one is used for the rest of the family. It's been really slowly lately and I want to reinstall xp on on the original harddrive. If I reinstall xp, will it change the ability to dual boot. Right now when we start the computer up it asks which drive/os we want to boot. Will I have to make any changes after reinstalling to have the same functionality?
Yes, you would need changes. However, just because your pc is slow does not mean reinstall XP. Have you cleaned Temp folders like in "C:\Documents and Settings\<user name>\Local Settings\Temp-Temporary Internet Files-History", "C:\Windows\Temp", browser temp locations ? When is the last time you did an HD defrag ? Sometimes the pagefile can be corrupt or too bloated, you can delete it in safe mode after a clean shutdown "C:\pagefile.sys". I'd recommend CCleaner as it has a registry cleanup mode too.