As far as i understood your query you are unable to select Xp at boot. Soln for this
Go to run>msconfig>boot increase the timeout setting and choose xp as the default OS.
If you want to reinstall xp pop in your Xp cd at boot. Ensure that your comp is set to boot using the cd drive. [to do this go to the BIOS by pressing del at startup] format the partition occupied by vista and install xp. WARNING: you WILL LOSE data.