<<I have setup software RAID tonight using the built in feature of Windows XP.>>
How did you do that? I'm interested to know, it looks like a very complicated procedure and you've to re-write some of the window files.( Correct me if I'm wrong)
I hope you know that when you setup Raid 0 with 2 different size HDD, the total capacity is the double of the smaller HDD. (In your case 20+20=40GB) The rest of your bigger HDD won't be used at all.