I've just installed XP Pro over Vista on a Toshiba laptop (Satellite Pro L40) and the only problem was that XP Pro doesn't come with Serial ATA drivers and you can only install them via a floppy drive

don't own one, don't want one!
Googling seems to suggest that your Asus has a Serial ATA drive, so you will have the same problem. I managed a workaround by creating an unattended install disk with nLite (
http://www.nliteos.com/) and including the Serial ATA drivers in the build. I've played around with manually creating unattended install disks for XP before, but this takes a fair bit of fiddling around and nLite seems to do a great job.
Also ckeck out
http://unattended.msfn.org/unattended.xp/view/web/1/ if you're interested in unattended installs; has saved me many weekends.
My experience with Vista so far, is that like so many upgrades in the past, MS have moved things around just enough so that I can't find what I'm looking for, or fix any problems that I have. Stick wih XP Pro; SP2 seems fairly stable (for an MS OS anyway!) and I believe SP3 should be with us soon.
Sláinte
midders
__________________
Hardware
Toshiba Satellite L10 Notebook, 1GB RAM, 80GB HDD
LG HL-DT-ST DVDRAM GSA-E10L external USB 2.0 (Firmware GSAE10L_LE07)
Software
Windows XP SP 3 (seems pretty stable so far...)
DVD Fab HD Decrypter, DVD Shrink, ImgBurn, Nero 6.6.1.15
Thoughts "I'd never belong to a club that would have me as a member" - Groucho Marx
"Some people bring happiness wherever they go, others whenever they go" - Oscar Wilde