The easiest way is described here: http://tortoisesvn.net/unversion.html
That article is a bit wordy and some of the methods didn’t work for me (like dragging the folder onto itself – Windows 7 wouldn’t allow it)).
Here’s what, in the article worked for me:
Simple: when you right-click on a
working copy and choose
“Export…” from the context menu,
a folder browse dialog is shown where
you can choose the folder where you
want the working copy exported to. If
you now select the very same path
that your working copy is on as the
target (i.e., you’re exporting the
working copy onto itself), TortoiseSVN
will remove all .svn folders of that
working copy.