Don,
I don't mean to be rude but what is the compelling reason for wanting to downgrade? Even if it works, it won't be trivial. Imo, if the issue is just 'look and feel' it won't be worth the hassle.
How many OS installs have you done before? Even without the restoring data issue, installing WinXP isn't for the faint hearted.
Looking at the Sony links that Halcyon posted above, I notice the system has a SATA controller and there is no XP driver on their site. Unless you can find an XP driver for that controller (Note - the Vista driver says it's an Intel Sata controller - the generic Intel Matrix driver can be found
here), XP is a non-starter. Even if you can find a driver, unless you have a USB floppy to load it during install you will need to slip-stream the driver into a custom XP install disk (using NLiteOS) - again not trivial.
As for backups, if you have a USB drive, just save the files you want on to there. Note - if you have used Vista's Windows Mail to store local messages, I doubt they will be compatible with Outlook Express.
In summary, unless there is a VERY good reason, don't do it.