TSQL -- Retrieve parent ID

TSQL -- Retrieve parent ID

The raw data in my table consists of an ID column and a column 'IsFolder'
which can be 1 or 0, like this:
ID IsFolder
1 1
2 0
3 0
4 1
5 0
6 0
7 0
8 0
9 0
10 0
11 1
12 0
13 0
14 0
15 1
16 0
17 0

The way the code interprets this is that if a line has a 1 for 'IsFolder'
then it is a Folder, and all of the tasks below it (until you hit the next
Folder) are children of that folder.
What I'd like to have is a Select statement that will just return the ID
of the parent folder for all non-folder tasks. So something like:
ID ParentFolder
2 1
3 1
5 4
6 4
7 4
8 4
9 4
10 4
12 11
13 11
14 11
16 15
17 15

I'm using MS SQL Server Management Studio 2005. I feel like this is an
easy answer to those familiar with using cursors (which I am not). I can't
think of any other way to do it but maybe someone else can. Anyway thanks
in advance for any answers and sorry if I did something wrong, this is my
first post.