Get the content by post id

Okay, I know this has been answered elsewhere, so a link is as good as an answer. (searched, but I did not find.)

I need to get a single post by ID, which is a variable.

Can I use:
query_posts("p=$newpostid");

It’s a bit fiddly..I’m using this in a popup called slideshow.php.

everything works but the part above, but fyi, what I’m trying to do is post the postid to slideshow.php (not index.php)– run the loop, putting the get_the_content into a variable.

then i run a bunch of regex thingies to pull out imagenames and captions, then makes a beautiful slideshow.