Have you installed SP1 for Vista? This was supposed to help with some of the speed issues. However, I am on XP, having experienced both, and not yet willing to deal with the issues surrounding Vista, as there are just too many. On another note, MS is offering free support for issues following installation of SP1 (see https://support.microsoft.com/oas/default.aspx?prid=11274&gprid=500921
), so it could be that, assuming that you can make them understand this is related to SP1, and that they could zero in on the reason for your speed issue, you might decide you want to stay with Vista. At any rate...
As for the mail, it sounds as though you have been using the Windows Mail included with Vista exclusively. There is not a "Windows Mail" program for Windows XP. The exact solution to the question of how to bring your mail back over depends on what you used previously, and what type(s) e-mail accounts you have/had. You could take a look at Windows Live Mail (it's free, at least for now) at http://get.live.com/wlmail/features#ID0E2BAC
and see if this would do it for you.
However, aside from the Vista/Mail/Live Mail combination, the solution for mail in going back to an XP OS depends on what you use and did use for mail previously. If you have an account with an ISP from which you are downloading your mail, you should be able to put copies of those messages back on the server by, in effect, e-mailing them to yourself if no other choices are available. (The individual choices / features depend heavily on the ISP and e-mail provider you are using.) I would say to just make sure you have things set in Windows Mail NOT to download the messages from the ISP before you start sending them to yourself.
Then, once you have reconverted back to XP and installed whatever program you will use to handle your mail (Outlook Express?), download them back in. You can export from Windows Mail or Outlook Express into Outlook - reference http://support.microsoft.com/kb/196347/en-us
- so this may represent another choice. The downside with this is that the date/time exported will end up being the date/time on the message in Outlook.
To summarize, no there is no Windows Mail - in XP, the only equivalent is Outlook Express. You will have to go to some effort regardless of the choice, but there are several ways to get your e-mail back down to the new choice. Best of luck, and if you need more info, please post back. Otherwise, thanks for rating FixYa!