This is the bug in Helios Service Release 2, Build id: 20110218-0911. This bug has been tracked here.
The comment/uncomment from shorcuts (Ctrl + /) and from menu (Source > Toggle Comment) does not working.
After some research, I found the solution. There is a patch.
All we have to do is download three files
Paste these files in eclipse/plugins/
Now, restart eclipse with -clean switch i.e. “E:\Softwares\eclipse\eclipse\eclipse.exe -clean”.
This fix worked for me and hope works for you as well.
Update : Url mentioned above are no longer accessible from sourceforge.net but they are still available on url Yes, I also found the links are missing. They might have been deleted from the sourceforge.net.
You can access all the files from http://download.eclipse.org/tools/pdt/updates/2.2/milestones/plugins/
Use search feature provided by browser to locate the url exactly. I was able to find all the files mentioned in my above post.