In my case, transferring the data I'd accrued using Vista's native apps back to my XP apps was the real problem. I'd used Vista nonstop for 60 days, and I retain all my e-mail correspondence with vendors to cover my, err, let's say posterior. Just copy it back? That's not so easy, as Microsoft decided to revamp (not for the better in my opinion) the way Vista and Windows Mail stores messages and contacts.
Where Outlook Express stored messages in database files and contact information in the single-file Windows Address Book, Vista and Mail store every e-mail and contact as a separate file--a more versatile approach but also considerably less efficient and inadvertently responsible for the auto-complete bug I mentioned up front. In addition, it makes Windows Mail very slow compared with Outlook Express.
Vista Mail's address export function worked fine for the contacts. Before reinstalling XP I exported to both vCards and a .csv (Comma Separated Values) file, which is basically a text file with one record per line and each bit of information separated by a comma.
While vCards seemed like a good idea at first, Outlook Express's vCard import function lets you add only one at a time, and if you drag and drop multiple vCards into the program window, you still have to click the Okay button for every card. With about 1000 e-mail addresses to copy, I soured on that path to carpal tunnel in short order.
CSV import was much faster, and with the exception of a couple of odd characters in the First Name field, the Mail and Outlook Express fields matched up perfectly, so I was able to restock the Windows Address Book in XP in about 30 seconds.
Two Ways to Import E-Mail into XP
Finding an easy way to move my e-mail back into XP proved more difficult. Outlook Express's import function won't import Vista Mail's .eml messages wholesale. The following method is simple and suffices--if you have a taste for the tedious.
You can save Vista Mail's messages to a safe place and then drag them directly into the Outlook Express window. But if you have a lot of sorted e-mail, you must manually recreate your folder structure--which is where the tedium commences. Here's the step-by-step:
1.  In Vista, save your e-mail repository to a safe place on another partition or drive before you install or overwrite Vista with XP. Find the repository in: C:\Users\Username\AppData\Local\Microsoft\WindowsMail\ Local Folders (Username is your user name). You can redirect the repository to another location by using the Tools, Options, Advanced, Maintenance, Store Folder function, or you can export in Mail format to another location. I recommend the latter (or both).
2.  After you've installed XP, open Outlook Express, open an Explorer window pointed to the safe location you copied the .eml files to, select them, then drag and drop them to the folder in OE where you want them. As I mentioned, you must recreate the folders yourself.
The above method works, but I have over 250 folders into which I sort e-mail or have archived for particular projects, so I really, really wanted a better way. Since I had Office 2007 with Outlook (any recent version of Office Outlook will do), I hit on the idea of importing the messages into Outlook from Mail under Vista, then transferring them back from Outlook to Outlook Express in XP. It's a slightly convoluted process, but it requires no tedious labor and works like a charm. The procedure goes thusly:
1.  In Vista, use Outlook's Data Management dialog box to create a new.pst data file, make it your default, and then import the messages (not addresses) from Mail. Make sure you save the resulting .pst file to a separate partition that won't be overwritten when you reinstall XP.
2.  In XP, install Outlook and use it to open the .pst file you created in Vista and make it the default.
3.  Fire up Outlook Express, choose Import, and select the Outlook option; you should now be good to go.
If you're wondering why I don't use Office Outlook for my e-mail, I have two reasons: First, Outlook Express is simple, it's extremely efficient, and I'm used to it. The other reason is that the Office version of Outlook allows only one set of contacts, which I use for the much smaller group of family, friends, and colleagues that I sync to my smart phone. And yes, I tried installing Outlook Express on Vista, but all the OE self-installers I possessed bailed after notifying me that a newer version (Windows Mail) was already installed.