First, did you back up your data before you installed Vista. If not
consider this to be a learning experiance. In the future, before installing
unstable and bug ridden operating systems, such as Vista Beta 2:
1) back up your data. Then after you have backed up the data, make an image
of your operating system drive.
2) come to the relevant newsgroup and spend several days reading before you
install the nasty bug ridden OS.
There are at least two ways to install Vista with one partition. The usual
way is to upgrade XP to Vista. If that is what you did, your data should be
in the Vista installation, and you should be able to back it up. Do that
first.
Now the bad news, if you upgraded Windows XP to Vista, then XP is gone, and
you cannot uninstall Vista. To get back to Windows XP you must back up your
data, reformat the partition and reinstall XP. Notice that I keep repeating
"back up your data". Find your data and back it up. If you can't find it,
or you don't back it up you are really screwed.
If you installed Vista on the same partition as XP then, you have gone
against Microsofts stern warnings, and you have a dangerously unstable
system far worse than just the Beta OS itself, but it is possible to remove
the Vista install and get XP working again, but most people have not had
good luck with this.
Unless you have a lot of time and really want to have a new learning
experiance, find your data, back it up, and reformat your partition.
Todd