I think the better idea is have 3 hd or 3 partitions.
1 - For windows, office, anti virus, all programs you need...
2 - For windows, optimized for audio just with you audio tools and DAW's, there you can remove the windows programs and services you don't need, leave in there just SO and audio requirements without tcp/ip, graphics for the best performance, without backup and secure systems, you can tweak all your windows xp with some appropriate tools like nlite.
3- For all your data audio data, docs, movies, mp3, pdf's everyting.
Anyway you can remove all your spyware, virus, disable services like system restore security center system, file indexing in all hard drives , do some stuff like search in goodle how to optimize windows xp, i found this site and it helps a a lot check here: http://mywebpages.comcast.net/SupportCD/OptimizeXP.html all the optimize tools you need are there.
Anyway if you have sata 2 and one pentium duo core, your machine will fly, and pentium quo core is coming
Raid is just for backup i think one home user don't need it, burn some dvd's with your important data and save one more hd for you do what you want
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