Questions about RAID setup

Hi guys. Complete RAID noob - as in never done it before so please bear with me if the qs sounds basic.

Basically I have a 4 year old comp atm with a 120gb Western Digital hd. I am going to build a brand new pc with a 250gb hd (can't remember make - will find it if necessary.)

I have XP installed on my current drive and obviously all my data. I want to keep this data and so stumbled across RAID and thought it may be the solution for me. I want to use Windows 7 on my new harddrive. My questions are as follows:

1. Can you use two hard drives in raid with different operating systems?

2. I will be using the 250gb primarily and so would log into windows 7 (I guess). Will I be able to able to use the data on the old xp drive or will it have a different encryption or something like that?

3. Will using the old drive slow my new comp down?

Thanks for any replies.